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Pressure drop and die surface changes in unplasticized PVC extrusion
Author(s) -
Szarvasy I.,
Pittman J. F. T.
Publication year - 2003
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.10126
Subject(s) - materials science , extrusion , surface roughness , pressure drop , composite material , surface finish , scanning electron microscope , drop (telecommunication) , die (integrated circuit) , analytical chemistry (journal) , nanotechnology , mechanics , chemistry , telecommunications , physics , chromatography , computer science
Pressure drops were measured as a function of time during extrusion of a commercial unplasticized PVC formulation through a specially constructed slit die. For a die with a wire‐eroded surface, the pressure drop per unit length is found to be higher upstream than downstream by about 21% at the start of extrusion, falling to 16% after 130 h, owing to a more pronounced reduction in the pressure gradient upstream. Die surface average roughness, measured mechanically, dropped from an initial value of 2.7 μm to 2.2 μm, independent of position in the die. The lower pressure gradient and the lower sensitivity of pressure drop to decreased surface roughness at the downstream position are believed to indicate a thickening of the slip layer in the flow direction. For a die with a sand‐blasted surface, roughness fell from 1.05 to 0.95 μm over 130 h, appearing to approach a lower bound of 0.5 μm. Atomic force microscopy, and scanning electron microscopy with energy‐dispersive X‐ray spectroscopy was carried out on the surface of inserts removed from the die surface after use. In scans 50, 20 and 10 μm square, the AFM showed roughness on the small scale increasing after use. Raised areas of the surface are smoothed, but covered with parallel striations of approximately 0.5 μm, which, it is believed, are caused by titanium dioxide particles. These raised areas are coated with the lead stabilizer and lubricant. Lower areas remain rough, and trap mineral particles. Very little chlorine is found, indicating that PVC does not adhere to the surface. It is concluded that polishing to below 0.5 μm is unlikely to be worthwhile.