The melt viscosity of thermoplastic starch has been investigated as a function of temperature, moisture content, and processing conditions. The effects of various low molecular weight additives have also been studied. Starch melts exhibit power law behavior over the range of shear rates studied. Melt viscosity decreased with increasing temperature and moisture content (MC). The power law index m increased with increasing temperature. The consistency K decreased with increasing temperature and increasing moisture content. Moisture content during the pelletizing step influenced melt viscosities measured after equilibration to different MCs. All additives studied except glycerol monostearate (GMS) significantly lowered the melt viscosity of starch, some more effectively than water relative to starch with 15% MC. Starch with GMS had viscosities essentially the same as, or slightly higher than, starch/water. This behavior may be due to the presence of unmelted helical inclusion complexes of starch and GMS. Starch formulations at 160°C exhibited melt visocosities similar to an LDPE of melt index 1.8.

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Urea, various glycols and mixtures of these were tested as plasticizers for gelatinized cornstarch. Mixtures of cornstarch, urea andor triethylene glycol, glycerol or propylene glycol were extruded into ribbons. Mechanical and structural properties of the ribbons were evaluated by tensile testing, DSC and NMR. Flexible ribbons were obtained only when enough urea or glycol was present to lower the glass transition temperature of the starch near or below room temperature. Ribbons containing high levels of urea were rather stiff, reflecting the low mobility of urea. Ribbons containing high levels of glycol were, in contrast, soft and weak. Samples extruded with mixtures of urea and glycols exhibited enhanced elongations at break. All samples became brittle at very low humidities and soft at high humidities. Extrudate von Yaisstiirke mit Hamstoff und Glykolen: Strukturmechanische Eigenschaftsbeziehungen. Harnstoff. verschiedene Glykole und deren Mischungen wurden als Weichmacher fiir gelatinisierte Maisstiirke gepriift. Mischungen aus MaisstBrke, Harnstoff und oder Triethylenglykol, Glycerol oder Propylenglykol wurden in Form von Streifen extrudiert. Die mechanischen and strukturellen Eigenschaften der Streifen wurden durch ZerreiOprobe, DSC und NMR bewertet. Dehnbare Streifen wurden nur erhalten, wenn geniigend Harnstoff oder Glykol vorhanden war, um die Glas-Ubergangs-Temperatur der S a k e in die Nilhe oder unterhalb der Raumtemperatur zu erniedrigen. Streifen mit hohen Hanrstoffgehalten waren ziemlich steif aufgrund der niedrigen Mobilitat des Harnstoffs. Streifen mit hohen Glykolgehalten waren im Gegensatz dazu weich und schwach. Extrudierte Proben mit Mischungen aus Harnstoff und Glykolen zeigten beim Bruch eine erhohte Lhgendehnung. AUe Proben wurden bei sehr niedrigen Feuchtigkeiten sprode und bei hohen Feuchtigkeiten weich.