Plastisols viscosity changes

Higher solution viscosity results from higher resin soUds, whereas an inerease in solvent volume reduces viscosity. Soluble resins (polymers) produce more pronounced viscosity changes than do insoluble pigments or plastic particles. A plastisol suspension (plastic particles in a liquid plasticizer) may have a medium viscosity at 80% solids, whereas a coating may be highly viscous with a 50% solid concentration. The specific solvent will also have an effect on the viscosity, depending whether they are true solvents, latent solvents, or nonsolvents. Refer to Refs. 1 and 2 for more detail. 

By selection of those chlorinated paraffins specifically developed for the PVC industry to match the properties of primary plasticizers, reductions in costs can be achieved without significant change in properties. However, certain aspects can be improved by the inclusion of chlorinated paraffin such as flame resistance, chemical and water resistance, low temperature performance, and the viscosity aging stabiUty in plastisols. 

The temperature where the puttylike plastisol changes to a dry crumbly solid, which is just above the temperature where the plastisol loses its identity as a fluid in the viscosity/temperature curve, he defined as the hot bench dry point. ... 

Compatibility of PVC with plasticizer (PI) also strongly influences the rheological and processing properties of plastisols 48>. The effects of the diverse plasticizers on PVC varies since some plasticizers (even well compatible with polymer), e.g. dibutylphthalate (DBP), may readily escape from the obtained materials and articles 17,23,43. Plasticizers which are poorly compatible with PVC (the so-called secondary PI ) may be used for the purpose-oriented change of viscosity properties of a system (fluidity increase) and saving the base PI component. 

Auxiliaries to decrease viscosity of PVC-plastisols improve the coating properties and deaeration of PVC-plastisols. Do not change the thermo-stability and light-fastness. 

Recycled PVC particles drastically increase the shear viscosity (even more when they contain fractions of smaller particles) as shown in Table 10.5 (viscosity of fresh plas-tisol at 2.1 Pa.s increased to 28.3 Pa.s when 10% recyclate with finest fraction was added). The viscosity of these plastisols was unstable (after 24 hom plastisol with 10% recycled PVC had viscosity of 781 Pa.s because recycled particles were either swollen or dissolved by the plasticizer). With lower content of recycled PVC, viscosity increases and aging process are not so prominent, but changes in viscosity are also observed. 

Change of Viscosity with Time for Selected Commercial PVC Plastisol Resins. 

The relationships between Pr and cj) have been derived for suspensions of monodispersed hard spheres in Newtonian liquids. However, most real systems are polydispersed in size and do not necessarily consist of spherical particles. It has been found that here also Simha s Eq. 7.24, Mooney s Eq. 7.28, or Krieger-Dougherty s Eq. 7.8 are useful, provided that the intrinsic viscosity and the maximum packing volume fraction are defined as functions of particle shape and size polydispersity. For example, by allowing with composition, it was possible to describe the Pr versus cj) variation for bimodal suspensions (Chang and Powell 1994). Similarly, after values of [q] and (j)i were experimentally determined, Eq. 7.24 provided good description for the versus cj) dependence of several multiphase systems, e.g., PVC emulsions and plastisols, mica-reinforced polyolefins, and sealant formulations (Utracki 1988, 1989). 

Fig. 7. Storage stability of plastisols (change in viscosity after 30 days).

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. 

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