Development of starch-based materials

Starch-based materials show potential as fully degradable plastics. However, the current applicability of these materials is limited due to their poor moisture tolerance and mechanical properties. Starch is therefore frequently blended with other polymers to make the material more suitable for special or severe circumstances. By varying the components of the blend and the process conditions, the morphology and hence the properties can be controlled. A clear understanding over the structure formation process will allow the development of new, biodegradable blends based on starch-based materials with better properties. The overall goal of this thesis was thus to develop insight in how the material (blend) properties depend on the processing, and based on this insight, explore new processing routes. Structure-function relationships: exploring a polymer science approach In Chapter 2, we discuss the relation between the performance of a plasticized starchbased film, in terms of permeation of volatile components, and its composition. Estimations of the Maxwell-Stefan diffusion rates of trace volatile components through plasticized starch films were developed based on free-volume theory and the Flory-Huggins-Maxwell- Stefan (FHMS) equation. The model correctly predicted the order of magnitude of the permeation fluxes of diacetyl and carvone through starch films. The results of this chapter show that blending of starch with hydrophobic polymers could be an effective way to improve the barrier properties of the film. In Chapter 3, the influence of alternative plasticizers (i.e., glucose and glycerol) on the gelatinization and melting of concentrated starch mixtures was studied, using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The results were interpreted using an extended form of the well-known Flory-Huggins equation. The chapter exemplified the possibilities of using theories that were traditionally applied to synthetic polymers, to biomaterials, in spite of their much greater complexity. This approach led to quantitative and qualitative understanding of the influence of small plasticizers of industrial relevance on the gelatinization and melting of starch. Comparing the Flory-Huggins model results with experimental results, showed that the approach is useful for interpreting and predicting the gelatinization and melting behavior of ternary starch-based systems. It also showed that since the experiments were complex, systems were often not in true equilibrium and other disturbing effects were easily encountered. Therefore, one should be cautious to use experimental results for characterizing the thermodynamics of gelatinization in multicomponent systems. Processing: the use of simple shear In Chapter 4, the use of simple shear as an instrument for structure formation of plasticized starch-protein blends was introduced. A novel shearing device was developed to explore the formation of new types of microstructures in concentrated starch-zein blends. This device was used to process different ratios of starch and zein (0–20% zein, dry basis) to study the influence of the matrix composition and processing conditions on the properties of the final material. Confocal scanning laser microscopy and field emission scanning electron microscopy showed that under shearless conditions, the starch-zein blend forms a co-continuous blend. Shear transformed this structure into a dispersion, with zein being the dispersed phase. The large deformation properties were examined by tensile tests in the flow and the vorticity directions; they could be described using a model for blends having poor adhesion between the continuous and dispersed phases. In Chapter 5, we studied the effect of compatibilization, i.e., improvement of the adhesion between the continuous and dispersed phases in starch-zein blends through the incorporation of a component having affinity for both phases. Aldehyde starch was synthesized by introducing a reactive functional group (aldehyde). This group then reacted in the blend with zein (and/or other components), forming a macromolecular compatibilizer in situ. The effect of this compatabilizer on the interfacial properties of the blend was studied using different zein ratios. The blends showed improved adhesion between the zein and starch phases compared to the blends described in chapter 4. The aldehyde starch however also influenced the properties of the starch matrix (higher viscosity, stronger molecular breakdown, browning), which indicates that indeed physical or chemical crosslinks were formed inside the starch matrix, but on the other hand posed a limitation for practical applicability. Chapter 6 presented the use of rise bran extract as a food-grade compatibilizer for starchzein blends. This material was extracted from rice brans using super-critical water, probably contains Maillard components and shows activity as radical scavenger, antioxidant and surfactant. The influence of rice bran extract as compatibilizer was compared with that of aldehyde starch by preparing blends under shear conditions. Field emission scanning electron microscopy showed that both compatibilizers improved the adhesion between the zein and starch phases. The mechanical properties of the blends compatibilized with aldehyde starch showed poorer mechanical properties after storage under controlled conditions, possibly caused by retrogradation of starch. The use of rice bran extract as compatibilizer however led to good compatibilization with good stability during storage. The good compatibilization by rice bran extract was suggested to be caused by polysaccharide-protein complexes, which are also responsible for its emulsifying properties. Application In Chapter 7, the conclusions of the preceding chapters were collectively interpreted. First, the use of a heuristic approach for the rational design of thermoplastic starch-based materials was described. Then the use of the ternary diagram for the system starch-waterglucose developed in Chapter 3 was used to evaluate alternatives routes for the intensification of the enzymatic hydrolysis of starch. Finally, future trends in the development of starch-based materials were presented following the insights obtained in this thesis. These include the use of established theories developed for synthetic polymers, further exploration of the concept of compatibilization of starch-based blends, and the development of new processing equipment dedicated to material structuring.