Implementation of Devolatilization

The foregoing equations show that devolatilization may be accelerated by acting on the diffusion coefficient, the thermodynamic equilibrium, or the interfacial area. 

The diffusion coefficient depends on the temperature of the system and on the concentration of volatiles. An increase in the temperature results in an increase in diffusivity and a decrease in viscosity, both beneficial for devolatilization. However, many polymers are thermally sensitive, so there may be a practical upper limit on the temperature to which the polymer may be exposed, as higher temperatures may degrade the polymer. Heat stabilizers have been used to enhance the thermal stabilization [35]. 

Recently the use of supercritical fluids in processing polymers has received much attention, because supercritical devolatilization has the potential for producing high-purity products with lower energy [42-44]. The high solubility of the volatiles in the supercritical fluid enhances the driving force for devolatilization. When the polymer is exposed to the supercritical fluid, it is swollen, and the free volume in the polymer is increased so that diffusion of the volatiles out of the polymer is enhanced. 

There exists a wide variety of devolatilization equipment. According to Biesen-berger [4], the processes can be classified into two main categories non-rotating or still equipment, and rotating equipment, in which devolatilization is enhanced by mechanical agitation. 

In those processes three different regimes can be distinguished [48]. Each of them has a different rate-limiting mechanism, which depends mainly on the difference between the equilibrium partial pressure of the volatile in equilibrium with the melt (Pi) and the total pressure within the vacuum chamber (P). This difference (Pi — P) is often called the degree of superheat , SH. 

---