The influence of a new processing additive (particles of organically modified nanoclays) on the processibility of polyolefins in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer. Ziegler Natta and metallocene polyethylenes and one polypropylene were tested using these two pieces of equipment. It was found that the nanoclay additive had a significant effect on the extrudate appearance of polyethylene. It eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.05 to 0.5 wt%). To explain the possible mechanism for the effect of the additive on the processibility of the resins, shear and extensional rheological measurements were carried out for the pure resins as well those loaded with the nanoclay additives. It seems that the presence of nanoclays suppresses the development of extensional stresses to such high levels that can cause melt fracture phenomena. Finally, it was found that the combination of nanoclays with traditional processing aids such as fluoropolymers produce an enhanced processing aid that can increase the critical shear rates for the onset of melt fracture to levels much higher than the individual constituents when they are used independently. POLYM. ENG. SCI., 45:1098–1107, 2005. © 2005 Society of Plastics Engineers

Flow instabilities manifest themselves as distortions on the extrudate surface (melt fracture). They are usually observed at high production rates in many polymer processing operations. Certain fluoropolymers/fluoroelastomers have long been used as processing aids for surface melt fracture elimination. Recent developments have shown that a small amount of boron nitride (BN) powder may successfully eliminate surface melt fracture and also delay the onset of gross melt fracture to higher rates. It has also been reported that a combination of BN and fluoropolymer/fluoroelastomer enhances the effectiveness of the polymer processing even further. The main objective of the present work was to measure the surface properties of a number BN powders, mainly surface energy, in order to gain a better understanding of its performance as a processing aid. Based on this study, it can be concluded that surface energy plays an important role in deciding the possible interactions between the processing aid, polymer melt and the extruding surface. It is observed that the lubricious nature of BN along with an optimum balance of its polar (non-dispersive) and nonpolar (dispersive) components of surface energy renders BN a successful processing aid in eliminating both sharkskin and gross melt fracture phenomena. Polym. Eng.

In this work, we studied the adsorption capacity of boron nitride (BN) for fluoropolymer and polyethylene (PE) to gain a better understanding of the interactions and the performance of BN and fluoropolymer, and their combinations as processing aids in the extrusion of Ziegler-Natta PEs. We found that BN has a relatively high adsorption capacity for both PE and fluoropolymer. As a result, simultaneous compounding of BN and fluoropolymer into the host polymer causes fluoropolymer to be trapped within the bulk of the polymer, and prevents the fluoropolymer particles from coating the die wall during flow. This limits the effectiveness of fluoropolymer and BN as processing aids. To avoid this interaction, we added fluoropolymer separately in a dry form just prior to extrusion. In this case, the synergistic effect of BN and fluoropolymer as a combination processing aid was evident. POLYM. ENG. SCI., 45:669 -677, 2005.

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