Extrusion of a hot polymer melt through a cooler die zone substantially increases the extrudate swell of some thermoplastics. This effect was examined for commercial samples of low‐density polyethylene, polypropylene, and polystyrene. Two conflicting effects come into play during extrusion of a thermoplastic. Colder melt temperatures promote increased extrudate swell, but the same conditions also facilitate molecular disentanglement and reduced melt elasticity and die swell. Since the extrusion process itself may affect the relation between die swell and melt temperature, laboratory‐scale measurements for the design of processes like blow molding are better carried out with small‐scale screw extruders than with capillary rheometers. For some applications it may be advantageous to use a polymer whose die swell is particularly responsive or unresponsive to die temperature variations. The procedure described in this article can be used effectively to monitor this characteristic.
This investigation deals with the nature and relative abundance of stable radicals formed by gamma‐irradiation of wheat starch at room temperature. Additions of equal weights of water, methanol, and ethanol were equally effective in reducing the content of stable radicals in starch which contained about 12% water before the additions. When, however, the starting material was dried starch with 2.9% initial water content additional water and methanol were better radical scavengers than ethanol. This difference is attributed to the superior ability of water and methanol to permeate the starch structure. Superficially different ESR spectra were obtained in products made by irradiating starch and starch that contained added water or methanol. Computer simulation of these spectra showed that they could be matched by superposition of the spectra of the same four component radicals, with some adjustments of relative intensities and peak widths. The structures of these radicals have been dedueed from the spectral assignments and relative effects of the three solvents used on the intensities of the respective ESR spectra.
Starch-g-polystyrene and starch-g-poly(methy1 acrylate) copolymers were synthesized and extruded without separating homopolymer PS or PMA. The extrusion behavior resembled that of thermoplastic melts in that these products were shear thinning and exhibited significant extrudate swelling and entrance pressure losses in capillary flow. These and other observations indicate that the materials comprise deformable polymer-grafted starch particles suspended in the corresponding homopolfmer melt. Flow is apparently by superparticle, rather than continuum melt mechanisms.
ZUSAMMENFASSUNG:Starke/Styrol-und Starke/Methylacrylat-Pfropfcopolymere wurden hergestellt und ohne vorherige Abtrennung von homopolymerem Polystyrol (PS) bzw. Poly(methy1acrylat) (PMA) extrudiert. Das Extrusionsverhalten ahnelte dem thermoplastischer Schmelzen darin, d& die Copolymeren die Scherung herabsetzen, eine merkliche Strangaufweitung verursachen und Eingangsdruckverluste im Kapillarflieherhalten auftreten. Diese und andere Beobachtungen deuten darauf hin, da13 diese Materialien deformierbare, mit dem jeweiligen Polymeren gepfropfte Starkepartikel in der entsprechenden Homopolymermatrix enthalten. Die FIieDeigenschaften ktinnen anscheinend eher durch solche ,,Superpartikel" als durch das Model1 einer kontinuierlichen, homogenen Schmelze erklart werden.
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