Flow‐induced crystallization of poly(ethylene terephthalate) melts in a capillary die: Formation and properties of the crystalline toroid

Abstract

An unusual crystalline aggregate, which is toroidal in shape, has been formed at the entrance region to capillary dies during the high‐pressure extrusion of poly(ethylene terephthalate) melts in an Instron capillary rheometer. The toroidal remnant was discovered in capillary dies removed from the rheometer after the occurrence of oscillating flow. Detailed observations of the rheological factors involved in the formation of the crystalline toroid were made. Examination of recovered entrance plugs disclosed that the crystalline toroid was a hard, stagnant zone which throttled the bulk flow of polymer melt into the capillary during the extrusion experiments. Hot‐stage optical microscopy, differential scanning calorimetry, and wide‐angle x‐ray techniques were used to initiate a structural study of the crystalline toroid. The toroid contained clusters of fibrous crystals which persisted to high temperatures. The fibrous clusters were randomly oriented in a matrix polymer which melted normally. Based upon the rheological and structural evidence, a mechanism of crystalline toroid formation is proposed.

Rheological Features and Flow‐Induced Crystallization of Branched Poly[ethylene‐co‐(1,4‐cyclohexanedimethylene terephthalate)] Copolyesters

“…The pioneering work about induced crystallinity in polyethylene in a extrusion capillary rheometer, published by Southern and Porter almost 40 years ago,52 was developed and extended to the case of PET by Cuculo and coworkers53–56 in the late 1970s and early 1980s. These authors focused on the role played by converging flows at the entrance of the capillary die, concluding that the formation of crystals should be attributed to elongational rate in the converging stream.…”

Thermal analysis

“…Hot-stage microscopy, DSC, and wide angle x-ray measurements were used to show that toroidal aggregate formed on high-pressure extrusion of polyethylene terephthalate) resulted from formation of fibrous crystals (136).…”

The elastic fracture of polycondensates

“…However, at temperatures above 275°C the oscillation phenomenon ceases to occur and extrusion remains stable at all flow rates. The behavior is somewhat sensitive to molecular weight in that a smaller-diameter die was necessary to induce pressure oscillations in PET of lower molecular weights; however, temperature is still the important condi- ( 5 ) Z i a b i c k i and Kedzierska ( 6 ) Mizutani and Arai ( 7 ) Mizutani and Arai ( 7 ) Griswold and Cuculo (8) tion. Other work in our laboratories in which a temperature gradient is applied along the length of a converging capillary die has shown that PET almost invariably fractures when the die temperature is close to the solidification temperature no matter what the other extrusion conditions.…”