Coating kinetics of fluoropolymer processing aids for sharkskin elimination: The role of droplet size

Multi-scale phenomena induced by fluoropolymer processing aids during the extrusion of linear-low density polyethylene

“…More recently, Lavallée [14] has presented pictures of fluoropolymer deposits on gold and nickel surfaces. Since the studies of Karchenko et al [15] and Bigio et al [16] in capillary rheometry, it is believed that fluoropolymer droplets are first adsorbed at the die entrance, and then migrate downstream to the die exit in a streaky fashion. These authors observed, by a technique of frustrated total internal reflection (frus-TIR), fluoropolymer deposition in the form of streaks of 200 nm thick and several microns wide.…”

Influence of die surface on the efficiency of fluoropolymer processing aids during the extrusion of linear-low density polyethylene

“…We demonstrated that wall slip was spatially heterogeneous and directly correlated to the local surface density of coating. Besides, we showed a mechanism of deposition different from the ones proposed in the literature [5,6]. Indeed, we observed that PPA droplets rather coated the die surface in a random way, with simultaneously aggregation and stretching of these aggregates, followed by a second overlapping mechanism of coalescence and development of PPA long streaks in flow direction.…”

“…It is thus of great interest to understand why there are PPA deposits on the die surface. The mechanism of coating is often discussed in the literature [5,6] but not the reasons of coating, except that the low surface energy of fluoropolymers might have an impact. We measured the static Young angle using goniometry (Goniometer DGD Fast/60 GBX, France), for comparing the work of adhesion of the different polymers on the steel die surface.…”

“…Other authors [3,4] confirmed the occurrence of wall slip by Particle Image Velocimetry (PIV) or Laser Doppler Velocimetry (LDV) techniques. Kharchenko et al [5] and Bigio et al [6] showed in a capillary sapphire die by Frustated Total Intern Reflection (Frust-TIR) that pressure decrease and sharkskin elimination appeared simultaneously with fluoropolymer droplets deposit on the die surface. By observing the evolution and the location of these deposits, the authors assumed that the deposits arriving at the die exit by migration enabled to eliminate the sharkskin defect.…”

“…Thanks to these new techniques and observations, it is now possible to get further information about the influence of process parameters in order to optimize PPA efficiency. Some studies have already looked at the influence of process parameters such as flow rate [9], or PPA parameters such as concentration [10] and particle size [6] on sharkskin defects.…”