Pelletizing melt fracture

Fig. 12.27 Surface morphological features of mLLDPE (ExxonMobil Exceed 350D60) extrudates obtained at 160 °C with a tungsten carbide die D — 0.767 and L — 25.5 mm just above and in the sharkskin melt fracture flow-rate region. [Reprinted by permission from C. G. Gogos, B. Qian, D. B. Todd, and T. R. Veariel, Melt Flow Instability Studies of Metallocene Catalyzed LLDPE in Pelletizing Dies, SPE ANTEC Tech. Papers, 48, 112-116 (2002).]...

Fig. 12.28 Melt-fractured pellets extruded at 8 kg/h per die hole (3.2 mm in diameter) from a conventional underwater pelletizing die (left) and smooth pellets extruded under the same conditions at 24 kg/h per die hole from the same underwater pelletizing die when locally heated to high temperature (right) (57).

Fluoropolymer-based additives are very useful, offering low temperature processability, chemical resistance, weatherability, self-extinguishing, and flexibility. Available as powders, pellets, dispersions or master batch concentrate (at additions of 250-1000 ppm), they are designed for use in PO film extrusion to eliminate melt fracture or problems arising from die build-up when running PO polymer with fillers and pigments [17, 59]. 

Suppose that 2% carbon black is mixed with PE in a melt extruder and that the product is pelletized and shipped in 25 kg bags. We randomly select 10 bags, extract a number of pellets, and fracture and analyze them in a microscope for carbon black particle statistics. The weight fraction of carbon black is calculated from the number of particles, and their size and density, and it was found to vary as follows in the 10 bags ... 

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