Flow Instability in LLDPE Processing and Its Control by Fluoropolymer Additives

Abstract Flow instabilities during the capillary extrusion of an octene-LLDPE have been measured by signals from an elongational rheometer used to wind up the extruding polymer filaments. The presence of fluoropolymers at concentrations above 400 ppm suppressed or eliminated the instability signal, but only after several minutes of extrusion. The time required to suppress instability was used as an indicator of additive effectiveness. Fluoropolymers were found to increase in effectiveness with increasing degree of polarity, as measured by acid/base interaction indexes and by non-dispersion surface energies. The relative apparent melt viscosities of host and additive polymer also were involved in effectiveness ratings. It is suggested that fluoropolymer additives suppress sporadic adhesive failure of the matrix polymer by forming an interphase between the extruder (die) wall and the flowing bulk polymer.