Structure and rheological properties of starch-PE compounds in film blown applications

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Polyethylene (PE) is known for its excellent waterproof properties, but it is permeable to oxygen and other gases. Starch is a good oxygen barrier, but is not water resistant. A good mixture of both materials should result in products with high water and oxygen barrier. The combination of PE and starch in a blend is currently not used for the production of films, but is potentially very interesting. An important advantage is that starch is produced from renewable raw materials such as potatoes, making the film partially bio-based. Gradually bio-based films can replace the existing petroleum-made films.

The present research focus in two main questions:
 How the rheology of a system containing polyethylene and thermoplastic starch can help us understanding the blend structure?
 How to measure the rheological properties of the blend components such that they are representative for the blending process and film structure?

Different methods have been applied in the literature to correlate the film blowing properties and morphology of thermoplastic starch based blends to basic rheological properties like shear viscosity and elongational properties [1]. However, the application of these methods to samples where the amount of water varies was not yet studied. Also, it has been shown in recent publications that elongational viscosity can give an indication of the co-continuity window of immiscible polymer blends and that strain hardening (in one or both components of the blend) expands these window of co-continuity [2]. In a blend where starch is present more parameters such as water content, presence of other plasticizers and plasticizer content play a crucial role. In practice, the main challenge is preventing that water evaporates during analysis and that analysis conditions are similar to the processing conditions.

There are different methods to measure the rheology of starch. In this study, three methods were used to characterize the rheology of starch: A rapid visco analyzer, a capillary and a dynamic rheometer. Each method provides different information that could eventually be correlated to the blending process or to the blend structure.
By using these methods the effect of glycerol as plasticizer in the rheology of the system was studied, results could be related to the extrusion process. Also, the effect of starch types with different rheology was studied and related to the morphological structure of the blend.

[1] Zullo, R and Iannace, S. The effects of different starch sources and plasticizers on film blowing of thermoplastic starch: Correlation among process, elongational properties and macromolecular structure. Carbohydrate Polymers 77 (2009) 376–383
[2] Hedegaard et al. Effect of extensional viscosity on cocontinuity of immiscible polymer blends. Journal of Rheology 59, 1397 (2015) 1397-1417.
Period10 Apr 201811 Apr 2018
Event title69th Starch Convention
Event typeConference
LocationDetmold, Germany
Degree of RecognitionInternational