Thermoplastic processing of starch and cellulose-containing polymers

Doctoral thesis, 2007

Starch and cellulose are two naturally occurring polymers that have the potential to reduce the dependence on fossil resources if used as replacements or as complements to synthetic polymers. There are however some problems associated with the use of these polymers that have to be addressed and solved. In this thesis, problems related to the processing of thermoplastic starch and cellulose-containing polymers are primarily in focus. Starch and cellulose are very similar in chemical structure but only starch can be processed thermoplastically, i.e. it can be melted and remelted again, provided that plasticizers, usually water and glycerol are present. Cellulose can on the other hand be used as a reinforcement in a synthetic polymer thus giving a composite material. Starch grades with different amylose contents and hydroxypropylated/oxidised starch were melt-processed together with glycerol and water as plasticizers. The processing techniques included extrusion, compression moulding and film blowing. The latter constituted a challenge but could be performed by optimising the processing conditions as well as the composition of the thermoplastic starch. The rheological behaviour of the starch melts were determined both in shear and in extension and the processed material was evaluated with respect to its mechanical and thermal performances. Further the influence of the processing on the morphology and the molecular structure was characterised. Cellulosic fibres with and without a clay coating were melt-blended with polypropylene or poly(lactic acid). The fibres were coated with non-delaminated or delaminated clay (nanoclay) with the aim to reduce the degree of fibre flocculation normally encountered with this type of systems. The morphology and the mechanical properties of injection-moulded or extruded composites were evaluated. In the case of starch processing, high amylose content materials were found to be more difficult to process but their mechanical performance was better. Though the melt tenacity of starch was quite low it was possible to produce films from starch by film-blowing. Moisture and glycerol content as well as temperature were identified as governing parameters for a successful processing of thermoplastic starch. The fibre flocculation in the cellulose-containing materials was reduced as a result of the coating of fibres with the clay.