Plasticization steps

Marcilla, J.C. Garcia and M. Beltran Chemical Engineering Department, Alicante University, Spain 

Most applications of plasticized materials involve their transformation from a solid paste, or liquid into a rubbeiy material by the effect of temperature. The case of plasticization of PVC is by far the most studied, and so this chapter deals with the steps observed during PVC plasticization. 

Plasticized PVC compounds can be obtained either fiom a plastisol (a liquid suspension of emulsion PVC in a plasticizer) or from a diy mixture of suspension PVC with plasticizers, obtained by diy blending. In both cases the behavior of these materials is quite similar as is the number of steps that have been described in literature. The interactions that occur between the PVC and the plasticizer as well as the changes in these materials caused by the effect of temperature are responsible for the behaviors observed. Titow described two stages during plasticization of PVC  

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. 

Steps one and two can take place at ambient temperatures after the plasticizer and PVC are mixed (aging). The third step can also take place at ambient temperatures, but the actual temperature depends on the plasticizer solvent power. For the fourth step to occur, which is the step with a high activation energy, it is necessary to heat the sample. The crystallization of small PVC crystals and formation of weak bonds happens during cooling. This step may take hours or days. 

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Steps 3 and 4, however, can be described as chemical plasticization since the rate at which these processes occur depends on the chemical properties of molecular polarity, molecular volume, and molecular weight. An overall mechanism of plasticizer action must give adequate explanations for this as weU as the physical plasticization steps. 

In the plastication step of the process, melting is critical in controlling cycle time. Also, during thermoforming, the heating of the sheet is the most time consuming step of the whole process. For example, if we consider the melting of an infinite slab, at an initial temperature of To, as presented in Fig. 6.61, the heat supplied by thehot wall, set at a heater temperature of Th, will create a layer of molten polymer of thickness, X (t). 

At first, the low molecular molecule attaches to the surface of the plastic (step 1 adsorption). If the molecule is then unable to penetrate into the plastic, no further... 

The screw is now positioned in the barrel with the correct measured charge of material between the screw tip and the nozzle end of the barrel. This plasticizing step occurs automatically in the press cycle such that it is completed by the time the mold is closed and ready for another cycle. 

Plasticizer polar groups interact with the polar groups on the resin— intermolecular plasticizing step. 

The structure of the resin is re-established, with full retention of plasticizer—intramolecular plasticizing step. 

Extrusion is one of the most important operations in the polymer processing industry. A substantial part of all polymers actually passes through an extruder at least onee in its production path from the polymerization reactor to the finished product. Most screw extruders in current operation are plasticating extruders. The plasticating step is usually the primary function of the plasticating extruders and has a profound effect on extruder performance. A fundamental understanding of the plasticating phenomena has been briefly provided in several books [1-2]. 

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