Optimizing Process Aid Performance by Controlling Fluoropolymer Particle Size

Fluoropolymer process aids (PPAs) are commonly used to eliminate melt fracture during the extrusion of polyolefins, and their performance may be quantified directly by measuring the fluoropolymer deposition rate on internal die surfaces, or indirectly by measuring the time needed for the PPA to eliminate melt fracture under a particular set of conditions. Historically, the performance of PPA has been considered optimal when the fluoropolymer enters the die in the form of small discrete particles, less than about 1 mm in diameter. Using both direct and indirect measurements, the present study shows that the reverse is true – performance improves when the PPA is more coarsely dispersed, so that fluoropolymer particles larger than about 2 mm in diameter enter the extrusion die. Based on these observations, a quantitative model of the fluoropolymer coating process is developed. The model fits the experimental data on fluoropolymer coating thickness as a function of fluoropolymer particle size and extrusion shear rate, and also provides insight on other aspects of the PPA coating process. These findings culminated in the development of the Z Technology ^TM PPAs, which use interfacial agents and a fluoroelastomer having a distinctly different rheology from the traditional fluoroelastomers in PPAs to deliver the large fluoropolymer particles needed to provide reliable, quick elimination of polyolefin melt fracture at low PPA usage levels.