Solution, Suspension and Casting Processes

As with thermoplastics melt processes, the setting is achieved by cooling. It will be appreciated that such cooling is carried out while the polymer is under stress so that there is considerable frozen-in orientation. This can be maintained throughout the life of the article. It is possible with the higher molecular weight materials to heat shapes made from blanks many years previously and see them return to the original shape of the blank. 

Although much less important in tonnage terms than processing in the molten and rubbery states, solution, suspension and polymerisation casting processes have a useful role in polymer technology. The main problem in such processes is to achieve a control of the setting of the shape once formed. 

Solution processes are used mainly for coating, film casting and fibre spinning, i.e. in processes where the distance the solvent has to diffuse out of the solution once coated, cast or spun is short. Regulation of this diffusion process is important if products of even quality are to be obtained. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Suspensions other than those of traditional latex type also exist. For example, particles of PVC suspended in plasticisers are widely used. After shaping, PVC pastes, as they are known, are heated to above the 7g of the PVC. At such temperatures the plasticiser diffuses into the PVC particles and this is followed 

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Esters of methacrylic acid are obtained directly from acetone cyanohydrin by reaction of the latter with concentrated sulfuric acid to give methacrylamide sulfate, followed by reaction with an alcohol. The process is continuous and the methacrylamide sulfate is not isolated. Acetone cyanohydrin is derived from acetone and hydrogen cyanide (Pig. 15-39), Polymerization Procedures. Of particular importmice to the acrylics is the cast or bulk method of polymerization. This method is employed to produce cast polymethyl methacrylate sheets which are widely used in industrial applications. Careful control of polymerization is required to obtain a bubble-free product with good optical clarity. A typical flow sheet for the production of cast eet is shown in fig. lfi-40. Solution, suspension, and particularly emulsion polymerizations are also, widely used with the acrylics. Such polymerization reactions involve relatively conventional batch-type processes. i... 

Preparation of films can be achieved by two main processes the "wet" and "dr) approaches. The "wet process" involves biopol5mier dispersion or solubilization in a film-forming solution (solutioncasting) followed by a second step, the suspension or film-forming solution is placed in a suitable mold and the solvent is evaporated under controlled conditions (Ciannamea et al., 2014 Hernandez-Izquierdo and Krochta, 2008). The result of the casting process depends mainly on the mixing conditions temperature, time, type and concentration of solvent, plasticizer and pH, and the dr nng relative humidity and temperature (Gallstedt et al., 2004). Most of the studies on polysaccharide-based films use this method because it is simple, reproducible in most laboratories, and useful as a first approximation to the formation of edible films. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts... 

Fabrication methods have overwhelmingly focused on improving nanotube dispersion because better nanotube dispersion in polyurethane matrix has been found to improve the properties of the nanocomposites. The dispersion extent of CNTs in the polyurethane matrix plays an important role in the properties of the polymer nanocomposites. Similar to the case of nanotube/solvent suspensions, pristine nanotubes have not yet been shown to be soluble in polymers, illustrating the extreme difficulty of overcoming the inherent thermodynamic drive of nanotubes to bundle. Therefore, CNTs need to be surface modified before the composite fabrication process to improve the load transfer from the polyurethane matrix to the nanotubes. Usually, the polyurethane/CNT nanocomposites can be fabricated by using four techniques melt-mixing (15), solution casting (16-18), in-situ polymerization (19-21), and sol gel process (22). 

Many studies have used these methods for processing of both thermosetting and thermoplastic polymers. Y. Liao (53) dissolved epoxy in a well-dispersed, ultra-sonicated CNT suspension. The solvent was evaporated, and the epoxy was subsequently cured to form a nanocomposite in which the good CNT dispersion was achieved. Jin et al. (54) produced various types of polymer-coated and polymer-grafted MWNT solutions, in some cases evaporating the solvent and subsequently melt-mixing with another polymer. Yudasaka et al. (55) used a mixture of SWNTs and PMMA in monochlorobenzene (MCB) for dispersion, purification and subsequent spin-casting of the material. 

In a plastisol process the gelation of PVC paste, which is a suspension of PVC particles in a plasticizer such as tritolyl phosphate, involves diffusion of the plastidzer into the polymer mass, resulting in a rise of the paste viscosity. Diffusion processes are involved in the production of cellulose acetate film by casting from solution, as casting requires removal of the solvent. Diffusion also plays a part in plastic molding. For example, lubricants in plastics compositions are required to diffuse out of the compound during processing to provide lubrication at the interface of the compound and the mold. Incompatibility with the base polymer is therefore an important criterion in the choice of lubricants in such cases. 

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