Development of starch-based materials

E.A. Habeych Narvaez

Research output: Thesisinternal PhD, WU

Abstract

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.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Boom, Remko, Promotor
  • van der Goot, Atze Jan, Co-promotor
Award date26 Oct 2009
Place of Publication[S.l.
Print ISBNs9789085854333
Publication statusPublished - 2009

Keywords

  • starch
  • mechanical properties
  • physicochemical properties
  • processing
  • polymer chemistry
  • biopolymers

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