Melt Processable Starch

In addition, newer aspects, such as the effects of sustainable materials based on starch on the macro or nanostructure and subsequent processing, thermomechanical properties and performance properties of plasticized starch pol5mciers have been examined (10). Specific structures and the resulting properties are controlled by many specific factors, such as filler shape, size and surface chemistry, processing conditions and environmental aging. In case of nanosized biocomposites, the interfadal interactions are extremely important to the final nanostructures and performance of these materials. 

A melt processable composition can be prepared by mixing starch urea, sucrose, and free water (1). The mixture is manually stirred to form a slurry. Poly(acrylamide) (PAAm) is dissolved in water solution. An aliquot of the solution is added to the slurry. Water in the slurry is then evaporated until the weight percent of PAAm in the final mixture is 0.2%. In another example, the PAAm was completely omitted. Typical compositions are shown in Table 5.4. 

A wide variety of other examples have been given elsewhere (11). 

It is generally known that melt shear viscosity is a material property useful for evaluating melt processability of the material in traditional thermoplastic processes such as injection molding or extrusion. For conventional fiber spinning thermoplastics such as 

the spinnability of the material can be determined simply by the melt shear viscosity, even though the spinnability is a property controlled primarily by melt extensional viscosity. The correlation is quite robust such that the fiber industry has relied on the melt shear viscosity in selecting and formulating melt spinnable materials. The melt extensional viscosity has rarely been used as an industrial screening tool. 

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It is well known that every starch processing method is facilitated by the presence of water. Water depresses the melting point of amylopectin crystallites and lowers starch melt viscosity. Native potato starch contains about 18% water when stored at ambient atmosphere (room humidity about 55%). If one does not add more than about 10% water the melt-processed starch will solidify when cooling down. If more water is added to the starch, its glass transition falls below room temperature (Zeleznak Moseney, 1987). The moulding then exhibits a rubbery state and solidifies later when the excess water evaporates. 

The effect of the type and amount of plasticizer on the mechartical, thermal and water-absorption properties of melt-processed starch was investigated [31], It was reported that, in general, monohydroxyl alcohols and high molecular weight glycols failed to plasticize starch, whereas shorter glycols were effective. 

In its natural form, starch is not meltable and therefore caimot be processed as a thermoplastic. However, starch granules can be thermoplasticized through a gelatinization process. In this process, the granules are disrupted and the ordered crystalline structure is lost under the influence of plasticizers (e.g., water and glycerol), heat, and shear. The resultant melt-processable starch is often termed thermoplastic starch (TPS). Since the advent of TPS,... 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

Starch is made thermoplastic at devated temperatures in the presence of water as a plasticizer, allowing melt processing alone or in blends with other thermoplastics (192—194). Good solvents such as water lower the mdt-transition temperature of amylose, the crystalline component of starch, so that processing can be done well bdow the decomposition—degradation temperature. 

Hot aqueous ethanol treatment CWS starches can also be formed by heating starch in hot aqueous ethanol24,92 93 or other solvents,94 including at atmospheric pressures.95 In this process starch crystallites are melted, but the water is insufficient to paste the granules. The solvent is then removed. The dried starch is stable. 

Other effective plasticizers for starch for imparting melt processibility include a variety of low molecular weight compounds, such as glycerol and diethylene glycol, and also polymers such as poly(ethylene-co-vinyl alcohol) [55]. Furthermore, starch plasticized in that manner can be melt blended with minor amounts of hydrophobic thermoplastics, such as polyethylene and poly( methyl methacrylate), to obtain biodisintegratable molded articles with good mechanical properties. 

The neoamylose co-crystallises with amylopectine from solution. In the network formed by cocrystallisation of the two polymers the tie molecules are the long branches of the amylopectin and the network-branches are the crystallites. Such gels can be made by an extrusion process, melting the starch and the new polymer and shaped into oriented, elastic fibers and films. 

Native starch cannot be treated as a traditional polymer because the arrangement of amylose and amylopectin leads to the already explained grain structure. As observed by Donovan [11] this grain structure has a melting process related to the amount of water contained in it. 

Based on conventional wisdom, such a starch composition would exhibit decreased melt processability and would not be suitable for melt extensional processes. However, the results reveal that the starch composition shows a significant increase in extensional viscosity when even a small amoimt of high pol5mer is added. Consequently, the starch composition is expected to have an enhanced melt extensibility and is suitable for melt extensional processes, such as blow molding, spun bonding, blown film molding, or foam molding (1). 

Attempts have been made to process natural starch on standard equipment and existing technology known in the plastic industry. Since natural starch generally has a granular structure, it needs to be destructurized and modified before it can be melt processed fike a thermoplastic material (1). 

Another method to improve the melt processability of starch are plasticizers that reduce the melting point of starch, such as dimethyl sulfoxide, polyols, amino, or amide compounds (10). The preferred plasticizer should have substantially a solubility parameter 5 of at least 15... 

Carvalho et al. [109] used ascorbic and citric acid as catalysts for the controlled hydrolytic cleavage of starch macromolecules carried out by melt processing in the presence of glycCTol and residual moisture. The process proved effective in providing a means for the controlled tuning of the molecular weight of starch in TPS compositions. 

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