AIR2-CT94-1187 Thermoplastic Starches for Industrial Non-Food Uses

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AIR Cluster VI - Bioplastics : Biopolymers/Gums : Fine Chemicals : Packaging : Paints/Coatings/Plastics : Starch

	Proposal No:	AIR2-CT94-1187
	Date Prepared:	November 1999
	Source:	Final report June 1999

Summary

The project was focused on the understanding of the concepts behind the well-known term "thermoplastic starch", in order to facilitate the development of specific products based on a thorough understanding of the material. Research within the project was sub-divided into the following tasks:

• Production and characterisation of starches and starchy fractions
• Optimisation of the melting process and analysis of its mechanisms
• Monitoring plasticisation and mobility properties
• (Bio)chemical modifications and blending
• Modelling of time dependent properties
• In-factory feasibility studies and application research

The results of these tasks were as follows:

Task 1 within this task, the production and characterisation of starch and starchy fractions was the major research item. The native and plasticised starches produced within this task served as standard materials for round robin tests or as large volume raw materials for industrial application tests. Starches were isolated via an optimised wet separation technique from smooth pea, wrinkled pea, wheat and maize. These starch-containing raw materials were also dry separated (dehulling, micronisation and air classification) in large quantities. In general, yields were > 60%, but could vary by 15% - 25%. Rye starch could be easily air classified. Yields were lower (25% - 40%) though the starch content could be as high as 85%. Within this task, plasticisation studies were also performed using the dry-separated fractions. It was shown (also within other tasks) that destructurisation of starch could be performed at temperatures below the melting temperature of starch using a high-shearing, low temperature extruder. Mechanical performance and observed molecular organisation of this type of destructurised starch were identical to high-temperature treated starch.

Task 2 Within this task extensive research was performed, aimed at an understanding of the melting process of starch. Several examples of interesting research results are as follows:

• A good theoretical description of the melting of model compounds of A- and B- type crystallinity (crystallinity present in native starches) was obtained using the first-order statistical thermodynamics (Flory theory). The specific interaction of starch polysaccharides with non aqueous polyhydroxy plasticiscers such as glucose, urea, glycerol etc. was well described.
• The factors affecting the occurrence of residual and recrystallised double-helical (A- and B- type) and single-helical (V_A-, V_H-, and E_H-type) crystallinity in various starches were determined using off-line and on-line (synchrotron X-ray diffraction) measurements. Also, the influence of B-type crystallinity on the mechanical performance was described.
• The rheological behaviour during extrusion of various low-hydrated, thermoplastic starches was analysed. The effects of water, glycerol, and amylose content on the viscoelastic characteristics of the material were determined. Also, effects of processing conditions such as shear rate on the viscosity were mapped out. The effect of the presence of a reversed screw element during extrusion, necessary for good destructurisation of starch, was modelled.

Although never complete, it could be concluded that research performed within this project on the melting of starch gave good and new insights in this relatively poorly understood phenomenon, while molecular and technological aspects, both extremely relevant for production of thermoplastic starch, were determined.

Task 3 Within this task the remarkable phase separation of the plasticiser glycerol and the starch polysaccharides amylose and amylopectin was analysed in detail. It was observed that phase separation occurred in both film casted and extruded materials, irrespective of composition. Ageing, coupled to restructurisation or recrystallisation, lead to an increased phase separation. Also within this task, the effect of amylose content (0 - 70%) on the mechanical performance of maize starch was analysed and oxygen permeability as a function of composition (such as amylose and glycerol content) and storage conditions (relative humidity, storage time) was obtained.

Task 4 Within this task activities were less coherent due to the large number of subtasks. Research extended from enzymatic modification during extrusion to fibre blending. It could be shown that esterification of starch can be achieved to a very high degrees of substitution which reduced the water sensitivity of the material. In addition, the reactive extrusion of potato starch in the presence of a -amylase was modelled. A good description of the effect of extruder type (single or twin screw), glycerol and water content and several processing parameters was obtained. The reinforcement of thermoplastic starch using fibres (O - 30%) from various origins was investigated. It was shown that the fibres reduced the observed ageing of the material.

Task 5 The ageing and biodegradation of processed starch was analysed in this task. A theoretical basis for the observed ageing was presented using the Tool-Naraynaswamy method. On the molecular level, the ageing was followed using X-ray diffraction, differential scanning calorimetry, atomic force microscopy and water sorption studies. Water mobility in these plasticised systems was monitored using desorption studies. Enzymatic studies were performed on model systems using amylose (isolated amylose and casted films) and starch-polyester blends. Amylose seemed to be able to partly resist alpha-amylase attack.

Industrial application tests were performed at various sites with mostly positive results. On the basis of comments from industry and industrial partners it could be concluded that main points of attention or current drawback of the material were its moisture sensitivity, which will limit its use to relatively dry applications. More extensive knowledge on blending or lamination, thereby reducing the moisture sensitivity, will broaden the market and should be part of future research.

Conclusion

This project increased understanding of starch plasticisation. Although not complete, it could serve as a sound basis for future product development.