Plastisols continuous coating

For the process of continuous plastisol coating by dipping in the range of velocities of practical interest, Masenko and co-workers 2,7,16 17> suggested the following empirical formula ... 

The pastes used are plastisols—finely divided polymer dispersed in plasticizer. They are mobile mixtures which may range in viscosity from liquids that can be poured readily to thick pastes—the viscosity depending essentially on the type of resin and the amount and type of plasticizer involved. Such mixtures can be applied to substrates by coating or by techniques akin to printing after application they are converted into homogeneous flexible vinyls by heating to temperatures in the range 175 to 200 °C, when the dispersed particles of resin dissolve in the plasticizer and are fused into a continuous structure.1... 

Metal powder processing techniques in which a preform is molded and sintered are used to process PTFE. Compression molding may also be used to fabricate PTFE parts. Its dispersions are applied by similar techniques to other coatings. Paste extrusion in which PTFE is blended with a hydrocarbon, prior to molding a preform, is used to continuously fabricate PTFE into tubes, tapes, and wire insulation. The hydrocarbon is vaporized before the parts are sintered. PVF is dispersed in a polar latent solvent such as dimethyl acetamide and is melt-extruded as a plastisol, followed by solvent removal by drying. 

Two of the principal methods of coating substrates with a polymer, namely extrusion coating and calendaring have already been dealt with in this chapter. Other methods of coating continuous webs include the use of dip, knife, brush, and spray. Dip coating, as applied to PVC, has already been described in previous section on plastisols. 

A plastisol may be regarded as an organosol in which the continuous phase is almost entirely liquid plasticizer (small amounts of solvent are used for viscosity adjustment). PVC plastisols are made from PVC powder, adipate and phthalate ester plasticizers and minor amounts of epoxy-type resin in solution to aid pigment dispersion and to help (with other additives) to keep the polymer stable to heat and oxidation. The resultant coating is nearly solvent-free and so can be applied in thick films (100-250 m) and stoved without disruption by escaping solvent. The plasticizer penetrates the particles, aiding sintering as the metal substrate reaches c. 200°C in 30-60 s. 

Plastisols have a variety of uses. They are ideally suited for spread coatings on continuous webs of fabric or paper by conventional knife or roller coating equipment. They may also be molded or cast into flexible shapes. 

In a variation of emulsion polymerisation, the process can be operated continuously, in which fresh VCM, emulsifiers and an initiator are fed into the reactor and PVC latex is withdrawn continuously. Such processes tend to require greater quantities of emulsifiers than the batch process. They produce latexes with a wide particle size distribution and thus low plastisol viscosities suitable for a wide range of applications, but the higher levels of emulsifier will preclude them from applications where water absorption or clarity of the final coating is important. 

Continuous emulsion Ability to produce polymers giving low plastisol viscosities. Product consisteiKy. High productivity. Coatings with poor water absorption and clarity (high residual emulsifier). Cannot produce pseudoplastic polymers. Inflexibility. Emulsifier costs. 

In the plastisol hot-dipping operation, a preheated mold is dipped into the plas-tisol for a predetermined amount of time. At the end of dwell time, the mold is withdrawn and the part is removed. As the process continues, the viscosity of the plastisol reduces considerably. Such a change in the flow properties of plastisol can have a significant effect on the appearance and thickness of the fused coating. The viscosity of the plastisol is conveniently measured by using a flow cup and any necessary adjustments in the viscosity are made. 

Expanded vinyl has been used as a pressure-sensitive tape backing for many years. Typically, a low-density foam is plastisol-coated on a casting liner that is used a number of times, or onto a continuous belt. After fusion and expansion, one or both sides are transfer-coated with adhesive that has been dried (and sometimes... 

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