Plastisol fusion rate

Plastisol Fusion—This property is a function of temperature, time, and solvating power of the plasticizer. This property may be measured by determining the clear point, that is, the temperature at which a clear solution is obtained when a low concentration of resin is suspended in a plasticizer and heated at a constant rate. 

Fusion Properties As with both flexible and rigid PVC, the higher the resin molecular weight, the slower the fusion. Plasticizer selection also plays a major role in plastisol fusion properties. PVC/vinyl acetate and other copolymer resin systems are sometimes used to speed fusion in plastisol systems. As the copolymer content increases, the rate of fusion increases. 

Propellants. Satriana and Bracuti (Ref 30) claim the use of pyrolyzed urea as a ballistic modifier which when incorporated in a doublebase plastisol propint produces a burning rate plateau. Thus, PbSnC>3 (90) and urea, acet and w (10g) are sintered for one hr at 500—50°, cooled, screened, molded and cured for use in a plastisol proplnt contg 30% HMX. Leach (Ref 37) reports that ballistic and other test results indicate that there is no significant difference between M30 proplnt contg Nitroguanidine prepd by the urea/AN process and that prepd by the standard British fusion process. 

A considerable range of plasticizers is available but those used most commonly are phthalate esters such as dioctyl or dinonyl phthalate (DOP or DNP). If it should be necessary to reduce the temperature of fusion, or to increase the rate of fusion (as with some expandable plastisols) a fast-solvating plasticizer such as butyl benzyl phthalate may be employed. Besides primary plasticizers such as these, less efficient secondary plasticizers (like chlorinated paraffins) can be included with a view to reducing costs and improving flame retardance. 

The rates of gelation and fusion processes of PVC plastisols are more critical in the operations involving simultaneous heating and flow of plastisol, such as present in rotational molding. It is because inappropriate plastisol viscosity (dilatant or too viscous plastisol) and a fast gelation process may alter required distributions of material, formation of bubbles and hydrocysts ° (see for example Figure 10.25). 

Figure 14.26. Evolution of elastic modulus with temperature for plastisols indicated until the gelation and fusion processes are concluded, (heating rate 10°C/min, frequency 1 Hz, shear stress 0.005, gap 0.7 mm).

The degree of fusion of the plastisol and the extent of wetting of the zeolite particles by the plastic matrix affects the rate of diffusion and extent of adsorption of methylene blue whic are preferentially adsorbed on zeolites. Experimental work indicated that addition of zeolites to PVC plastisols created an interface accessible to aquous solutions. 

Moiding. This is used for producing hollow articles such as halls, dolls, and toys. The process requires a plastisol with low viscosity at low shear rate that has a short gelation time and is easy to deaerate. In rotational molding the plastisol is poured into a cold mold that rotates aroimd two perpendicular axes whilst entering an oven where it is heated with air to 200-250°C. After gelation and fusion, the mold is cooled in a waterbath. 

G l3tion and Fusion En route to fusion, the plastisol passes through a gel state. Gelation is the point where all of the liquids have been absorbed, forming a soft solid. Gelation rate is important in applications where this state, dependent on time and temperature, determines the thickness of product. A common method is to run Brookfield viscosity versus lime in a heated oil bath, for example at 100 °C. Viscosity is plotted as a function of time, reaching a plateau on gelation. 

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