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PPA (Polymer Processing Aid) : A boost for many extrusions processes and many polymers.

SpecialChem / Sep 7, 2001

The general behaviour of a melted polymer flowing into a die is represented in Figure 2 : Melted polymer is in Yellow and die walls (1mm gap) are in Blue. Velocity profiles of the polymer have been reported inside and outside the die. As you can see, inside the die the speed of polymer's particles are not uniform. Maximum velocity is observed in the middle of the gap and reach almost zero at the walls. When the polymer is out of the die, the velocity profile is becoming flat and all the speeds are equal to the average speed observed inside (few mm over the die). On the specific point where the polymer is leaving the die, and for particles close to the wall, the speed is increasing rapidly from zero, up to average speed of the melt. This is creating an acceleration of the melt on short length equivalent of high shear stress. Elongational stress of LLDPE is over its possibility, and the melt breaks continuously at this place creating the melt fracture phenomena.Die build up is a deposit of burned polymers and fillers at the exit of the die.

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