PROCESSABILITY OF POLYPROPYLENE
Processability of a polymer is highly dependent on its rheological properties, which have close relationship with its molecular weight, molecular weight distribution, temperature and shear rate. PP resins are generally categorized according to their melt flow rates (MFR), which is the amount of material that comes through a standard die hole for ten minutes. Polymers with higher molecular weight have lower MFR and higher viscosity (under a given temperature). Commercial polypropylene has a wide range of MFR from 0.25 to 800. MFR is a very important parameter for both melt-blown and Spunbond processing. PP melts exhibit non-Newtonian viscosity, normal stress in shear flow, excessive entrance and exit pressure drop, die swell, melt fracture and draw resonance. PP melts are more visco elastic than PET and nylon melts. The flow pattern and stability of PP melts are highly dependent on the shear rate. Above the critical shear rate, melt fracture may occur.
Processability of polypropylene fiber is also influenced by the dye geometry. The L/D ratio has to be optimized to reduce instability and the effects of PP's high viscosity. Both melt fracture and draw resonance represent instabilities in flow. Draw resonance is a periodic variation in diameter of a spinning threadline above a critical draw down ratio. Slowing down the drawing operation or a suitable cooling procedure may prevent this.
In addition, processability of polypropylene fiber can also be affected by other factors such as finishing. Finish oil is a mixture of several chemicals that function as anti-static agent and lubricator to protect the filament. The results of the effects of finishing on this area are not available in this report yet.