Sheet Forming and Film Casting

Polymer flat him sheets are formed continuously by extruding a polymer through a more or less rectangular sheeting die, which is quite wide with a small opening. Because the extruder outlet is by necessity circular, and the die rectangular, two fluid particles feeding 

Additionally, the role of the relaxation region is to help erase the width-dependent upstream flow history. Thus the design and the choice of the flow passages from the extruder to the die per se are of great importance. A number of sheeting-die designs representing different practical as well as theoretical solutions are currently in use. 

Upon exiting the die, the sheet extrudate will swell to a level determined by the polymer, the melt temperature, the die length-to-opening ratio, and the shear stress at the die walls. Additionally, flow instabilities will occur at values of the corrected shear stress at the wall, of the order of, but higher than 105 N/m2, as found by Vlachopoulos and Chan (58), who also concluded that, for PS, HDPE, and LDPE, the critical Sr in slits is 1.4 times higher than in tubes of circular cross section. Aside from these differences, the information presented in Section 12.1 and 12.2 applies to slit flow. 

Polymer sheets are cooled without stretching by convected cold air (or an inert gas), by immersion into a fluid bath, or by passage over chilled rolls. Flat films are usually stretched and oriented uniaxially and cooled by either of the methods previously mentioned. Films are also cast and cooled on rolls for optimal clarity purposes. 

Sheet die design equations were first developed by Carley (59) for T-shaped dies using Newtonian fluids. Pearson (60), whose basic approach we now elucidate, extended the design equations to Power Law fluids. The proper die design delivers a given polymer melt under specified conditions through a constant die opening at a constant rate and temperature (cross-machine direction uniformity). Here, we trace the development of a die 

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Clear, water-soluble, oU-and grease-resistant films of moderate strength can be cast from hydroxyethylceUulose solutions. Elexible, nontacky, heat-sealable packaging films and sheets can be produced from hydroxypropylceUulose by conventional extmsion techniques. Both gums can be used in the formulation of coatings, and both can be used to form edible films and coatings. 

The first composite reverse osmosis membrane reported in the technical literature was developed by Peter Francis of North Star Research Institute in 1964 (4). This membrane was formed by float-casting an ultrathin film of cellulose acetate (CA) upon a water surface, removing the membrane from the water surface by lamination onto a pre-formed microporous support film and drying to bond the membrane to the support. This float-casting procedure has since been described in the technical literature for both flat sheet and tubular membranes ( 5, 6, T). 

Ionics CR-61 membranes are cation-selective membranes comprising sulfonated copolymers of vinyl compounds. The membranes are homogeneous films cast in a sheet form on a synthetic cloth backing. Thickness of these membranes is 20-25 mils. Ionics Bulletin No. CR-61.OC gives the properties and characteristics of aU Ionics CR-61 membranes. 

By the mid-80s it was clear to most researchers that success on the conductivity side had taken its toll on polymer processability. Attention turned back to restoring the solubility and mechanical properties of the polymer. Polyaniline received the most attention initially. The nonconductive emeraldine base form is soluble in A-methylpyrrolidone [28] and films can be cast. Subsequent doping with a protonic acid from aqueous solution, or in situ with a photo-acid generator [45], is necessary to achieve conductivity. Polyaniline is also soluble in sulfuric acid, not the most convenient of solvents. Nevertheless it proved possible to spin fibers [46], cast films and extmde sheets of conductive polyaniline sulfate, but the laboratory experiments did not make the transition into large-scale manufacmring. 

Cellulose acetate is the most important cellulose ester and various processing techniques for forming shaped bodies exist for this polymer. The mixed esters discussed above, CAP and CAB, are used predominantly for injection moulding applications in suitably plasticized form and are marketed as pellets. This is done for CA as well, but additionally, alternative processing routes are available where solution methods play an important role. This is in particular the case for fibre spinning and (thinner) film manufacture where cellulose diacetate (CDA) is dissolved in acetone and dry spun or solvent cast. Moreover, the traditional solventusing block process is employed for obtaining sophisticated, multicoloured thick sheets for applications like spectacle frames. 

The effect of casting solvent on the miscibility behavior of silk fibroin/PVF blends was investigated by Um et al. SF/PVA blend films cast from aqueous and formic acid solution. The p-sheet conformation of SF formed by formic acid casting was retained for aU SF blends regardless of blend ratio. SF/PVA blends from aqueous solution exhibited a phase-separated morphology and immiscibility by SEM observation and DMTA (Dynamic Mechanical Thermal Analysis) measurement (Um and Park 2007) (Figs. 10.35 and 10.36). 

Processing methods extrusion, injection molding, blow molding, thin-wall injection molding, injection stretch blow molding, sheet extrusion, blown fihn, cast film, rotomolding, and thermo-forming... 

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