(217eh) Effect of Magnesium Stearate On Processability and Properties of Thermoplastic Starch

Starch is one of the promising materials, which is interesting for biodegradable packaging applications due to its cheap, biodegradability, composability and renewability.  In some cases, starch is converted to thermoplastic material called “thermoplastic starch” or “TPS” by using a thermo-mechanical process along with the incorporation of plasticizers, e.g. water, glycerol and sorbitol.  TPS can be shaped into various profiles through conventional converting processes, such as cast sheet extrusion, injection molding, compression molding, blown film extrusion, etc.  Although the converting of TPS to sheet and film using cast sheet and blown film extrusions has been reported, its processability is poor due to the low melt strength.  In addtition, the obtained TPS film is not environmentally stable owing to its high moisture sensitivity.  Many additives, such as nanoclays, natural fibers, citric acid, fatty acid, etc. have been incorporated into TPS to facilitate its process and reduce water/moisture absorption.  Magnesium stearate, a compound consisting of magnesium and stearic acid (octadecanoic acid), is one of the candidates to be used as an additive for thermoplastics.  In general, magnesium stearate is widely applied as gelling agent, anti-sticking agent, stabilizer, lubricant, emulsifier and plasticizer for paint, food, polymer and pharmaceutical industries.  Previously, magnesium stearate has been added to TPS only for improving extrusion and injection molding processability of TPS.  However, the effect of magnesium stearate on TPS properties has not been reported yet.  Therefore, the objective of the present work is to study the influence of magnesium stearate on processability in cast sheet and blown film extrusions as well as on properties of TPS.  TPS was compounded with magnesium stearate in a twin-screw extruder.  The amount of magnesium stearate was varied from 0.25 to 1.5 parts per hundred parts of starch.  Incorporation of magnesium stearate not only improved cast sheet and blown film extrusion processability of TPS, but also enhanced its tensile properties (tensile strength ~ 20-160 %, modulus ~ 110-670 % and elongation at break ~10-20 %).  In addition, water vapor barrier properties of TPS became better (~10-20 %) by adding magnesium stearate.  However, incorporating magnesium stearate caused deteriorated oxygen barrier property and increased melting temperature of TPS (~ 10-20 °C).