Due to environmental concerns and the limited availability of oil, there has been an increasing interest in using starch as a substitute for petroleum-based plastics. Starch is advantageous due to its low cost, availability, and compostability. However, the complex multiscale structure of raw starch granules makes it difficult to produce starch-based materials, and thermal processing with a plasticizer, usually water, is required to disrupt the granule structure. Control of rheological behavior during processing is important to avoid flow-related problems, but the rheological characterization is challenging due to issues related to sampling and high melt viscosity.
Structures of Native Starch
Starch is an important energy and carbon storage molecule found in various plant parts. Starch granules come in different shapes and have varying crystallinity levels depending on the plant source. Starch granules have a complex, multi-level structure ranging from macro to molecular scales and consist of two major biomacromolecules, amylose and amylopectin. Amylose has sparse branching, while amylopectin is highly branched. Starch granules also contain small amounts of proteins, lipids, and phosphorus, which can interact with the carbohydrate chains during processing and modify the behavior of starch-based materials.
From Native Starch to Plasticized Starch
The process of gelatinization is essential in transforming native starch into a processable material. Gelatinization involves granule swelling, loss of birefringence, and solubilization of molecules, and it is irreversible. The process can be characterized using various techniques, including differential scanning calorimetry (DSC). When subjected to shear treatment, the gelatinization behavior of starch displays significant differences due to intense shear fields within the extruder. Variations in physical properties can occur both before and after the thermal transition, and the use of shear treatment can provide practical information for extrusion processing. Nonvolatile plasticizers, such as glycerol, are used in practical processing to lower the Tm of starch.
Read more: From Native Starch to Plasticized Starch
Rheology of Plasticized Starch Melts
The rheological properties of plasticized starch melts, including their steady shear viscosity and elastic properties, are important for understanding their behavior during processing techniques such as film blowing and foaming. The steady shear viscosity of plasticized starch melts is primarily measured using a capillary/slit die viscometer, and an increase in temperature or plasticizer content results in a lower viscosity. The elastic properties of plasticized starch melts can be characterized by analyzing the entrance and exit pressures in a die viscometer to measure the extensional viscosity and first normal stress difference. The Trouton ratio is commonly used to evaluate the elasticity of a material, and high Tr values indicate viscoelastic fluids. Understanding the elastic properties of plasticized starch is important because they are closely related to certain processing techniques such as film blowing and foaming.
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Starch Processing at Laboratory Scale
To produce biodegradable and renewable starch-based films and sheets, the process of solution casting involves solution preparation, gelatinization, casting, and drying. The low viscosity of the solution makes it suitable for preparing starch-based nanocomposites through solution casting, which can be used for food protection due to their oxygen-barrier characteristics. Plasticizers play an important role in processing and improving the mechanical properties of starch-based films. The article also mentions electrospraying as another method for preparing starch films. The use of starch-based films has the potential to significantly reduce the environmental impact of packaging materials.
Read more: Starch Processing at Laboratory Scale
Semi-Industrial Technologies Adapted for Starch Processing
Extrusion is a widely used manufacturing process in which a material is forced through a die to create a product with a fixed cross-sectional profile. This technology is employed in various industries, including food, plastics, and pharmaceuticals, to produce a range of products such as pasta, plastic films, and medical implants. This article provides an overview of the key aspects of extrusion technology, including the basic components and workings of an extruder machine, strategies and issues involved in the extrusion process, and different extrusion techniques used for specific applications.
Compression molding is a process that can be used to produce foams and containers from plasticized starch-based materials, and recent research has shown that microwaves can also be used for this purpose. The selection of mold material, pretreatment of starch, and initial loading of samples are important factors in achieving a uniformly foamed block. Mold-releasing agents are also added to prevent sticking. A simple melt compression molding process has been introduced to produce highly flexible starch-based films. The use of ionic liquids as plasticizers has also been explored and found to have a stronger effect on starch granule disruption and plasticization compared to glycerol.
Injection molding is a challenging process for plasticized starch-based materials due to their high viscosity and poor flow properties. Researchers have studied the behavior and processing parameters of starch injection to overcome these challenges, including adhesion to the mold, distortion, and shrinkage of parts.
In the future, there will be a focus on enhancing the processibility and material performance of plasticized starch-based materials. The development of innovative extrusion-based processing techniques, exploration of new plasticizers, and greater emphasis on rheology are crucial to achieve this goal. Starch-based blends and (nano)composites are important for utilizing starch as a renewable resource to the fullest extent. However, their development must be integrated with the advancement of melt processing techniques, and dispersion of nanoparticles may only be easily achieved with solution methods. REX offers a promising method for producing high-performance starch-based blend and (nano)composite materials.