Gelation and fusion

Kaolin has a use in plastisol formulations giving good rheological control. Calcium carbonate is also used in this area. An alternative to calcium carbonate, phyllite, has been evaluated. This material, which contains quartz, muscovite and kaolinite, was characterised on the basis of paste viscosity, gelation and fusion behaviour, and mechanical properties (70). 

Reasons for use improve compatibility with other additives and polymers, improve flame resistance, improve low temperature performance, increase chain mobility, increase filler loading, increase flexibility and elongation, inflnence blood compatibility, lower glass transition temperature, lower viscosity, lower processing, gelation and fusion temperatures, modify foaming rate and microcellular stractirre... 

During gelation and fusion some intermediate steps have been described. " The number of steps and temperatmes at which they occur vary slightly from one author to another depending on the type of experiment carried out. From the work of many researchers. Sears and Darbyidentified six steps which are described below. 

STUDIES OF PLASTISOLS BEHAVIOR DURING GELATION AND FUSION... 

Studies of plastisols behavior during gelation and fusion... 

Figure 9.2. Schematic behavior of elastic modulus and viscous modulus for plastisols with different concentration of plasticizer Cj < Cj < Cj. [Reprinted from European Polymer Journal, Vol. 34, Marcilla A., Garcia J.C., Qualitative Model for Viscoelastic Measurements during Gelation and Fusion of PVC Plastisols, p. 1341-1348, 1998, with permission from Elsevier Science].

Along with the progress of gelation and fusion processes, the relative proportion of phases (1) and (3) diminishes wliile phase (2) increases. When the material is completely gelled and fused, only phase (2) exists, and so just one single glass transition temperature can be observed. 

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). 

Gelation and fusion of PVC plastisols can be studied by different techniques classified as statical and dynamic methods. Among the dynamic methods, rheological characterization has shown to be a reliable technique of study. Using dynamic methods the influence of plasticizer concentration and type and the influence of the resin type (characteristic molecular weight and particle size distribution) were analyzed. The most relevant findings are discussed below. 

The stu of the plastisol curves with different types of plasticizers (phthalate family only) revealed that gelation and fusion processes take place faster in plastisols which contain plasticizers of lower molecular weight (more compatible plasticizers with PVC). Rheological properties changes according to this relationship (see Figure 10.27) DBP>DHP>DOP>DINP>DIDP. 

In addition to the factors cited previously, which could modify flow properties and gelation and fusion processes of PVC plastisols, the presence of recycled PVC particles could also provoke relevant changes, as previously reported, because of addition to fresh plastisols of preprocessed PVC particles with different particle sizes (four different fractions with diameters below 0.500 mm) in concentrations ranging from 0 to 10 wt%. 

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).

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