Implications for Processing Extrusion

In the continuous method of extrusion, CO2 is fed into the polymer melt and nu-cleation (and hence foaming) is initiated at the exit die. Pressure and temperature conditions at the exit die are controlled to result in supersaturation of the polymer. The inter-relationships between the key variables to control cell nucleation and growth in the continuous extrusion foaming process were summarized by Tomas-ko et al. [19]. Extrusion of polymers under CO2 pressure enables operation to occur at reduced temperatures, facilitates the blending of polymer blends, and provides an environment for reactions to occur (reactive extrusion) [105]. 

Lesser [88] studied the continuous extrusion of a range of high-melt-viscosity polymers using a single screw extruder with a temperature-controlled die. Control of the foam morphology (cell density and cell size distribution) of polytetra-fluoroethylene (PTFE), fluorinated ethylene propylene cofxtlymer (FEP), and 

Siripurapu et al. [60] studied the continuous foaming of highly crystalHne poly (vinyUdene fluoride) (PVDF) at 175 °C with 2 wt% of SCCO2, which resulted in a foam structure with a heterogeneous cell size distribution and a low cell density, attributed to the low solubility of CO2 in the polymer and therefore tittle CO2 available for bubble formation. A further restriction is the semi-crystaUine structure of PVDE. As the temperature falls below the crystallization occurs, which expels CO2 into the amorphous domains and may lead to cracking of the cell walls and therefore formation of discrete particles. Improvements in the foam characteris- 

Areerat et al. [98] developed a capillary rheometer equipped with a foam extruder to measure the viscosity of a LDPE/CO2 system. Tlie design included an NIR sensor to monitor the CO2 concentration within the polymer during processing. A schematic diagram of the equipment is shown in Fig. 10.6. Viscosity reduction was caused by an increase in CO2 concentration, with 5% CO2 corresponding to a viscosity reduction of 30% compared to that of the neat polymer. 

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