In the previous parts of this series, we have introduced a basic understanding of the market of bioplastics and subsquently illustrated the processing of a variety of plastics that are currently available in said market. Earlier we covered one of the most widely available plastics, namely PLA, and now we thread upon a similarly widely available plastic produced from starch.
Starch is a widely available and cheaply produced polysaccharide that is found in plant-based sources, such as agricultural produce. The wide availability is the result of its global occurrence as the second most common component in biomass, after cellulose. The polysaccharide’s monomeric building blocks consist out of glucose that is subjected to condensation and subsequently formed in two types of orientation, namely linear amylose and branched amylopectin1. The most potent commercial sources for obtaining starch suitable for application as a biodegradable polymer can be found in grains, such as wheat, maïs and rice, but also in tubers such as cassava and potatoes. When starch is present in biomass it occurs in the form of granules with sizes between 1 and 110 microns2. The source of the starch is critical in determining the properties of a particular starch, since it determines the polysaccharide molecular lengths and the amylose and amylopectin ratio3.
The properties of linear amylose do closely resemble the properties of synthetic polymers, though the chain length is oftentimes 10 times as extensive. The branching of the amylopectin reduces the mobility of the polymer chains and causes a high viscosity. Additionally, the ratio between amylopectin and amylose content is an important indicator for the crystallinity of starch4. And as a result the crystallinity of the starch granules is typically found to be in a wide range of 14 – 45%. The crystallinity is determined by the tendency of the amylopectin to take the form of helices due to the interaction of two chains. These chains are then oriented in the same direction, which forms the crystalline regions of the starch granules. A higher content of amylose disrupts the regular orientation of the helices and forms amorphous regions, which reduces crystallinity6.
Table 1: Starch sources, content and crystallinity (adapted5)
When products are made out of natural starch they suffer from brittleness, low resistance to acid and a high hydrophilicity. Additionally, the decomposition temperature of natural starch is lower compared to the melting temperature7. It’s difficult to give a precise temperature at which melting of natural starch occurs, especially since it is dependent on relative crystallinity also, but the range for perfect starch crystallites without water is measured to be between 160 – 210 ˚C8. The decomposition temperature is similarly dependent on the crystallinity and in a range between 50 – 120 ˚C9,10. The brittleness and the decomposition temperature are both characteristics that make the use of natural starch as a polymer difficult. It is also necessary to counter the retrogradation that is observed when heated starch is cooled. Usually the starch reverts to a higher crystalline state than the crystallinity detected before heating11. However, there are methods to combat the problems encountered with working natural starch:
- Cross-linking is a chemical modification methods, where hydroxyl groups are replaced with bonds linking two strains of amylose or amylopectin. This reinforces the starch to be more resistant to heat, shear and retrogradation and causes it to be more viscous and less soluble in water12,13. More substitution with bulky functional groups onto the amylose and amylopectin chains is oftentimes utilized to stabilise the material further. These functional groups then prevent further retrogradation. This also allows processing at lower temperatures7.
- Esterification presents a method to reduce the hydrophilicity by the reduction of the amount of hydrogen bonds. This is achieved by replacing hydroxyl groups by acetylation with hydrophobic functional groups, usually at a very low degree of substitution. The result is a product that is more heat stable and more resistant to retrogradation. At higher degrees of substitution the starch becomes soluble in organic solvents opening up the possibility to perform film casting11,14.
- Conversion of the native starch to a form with a lower molecular weight is also a possibility for modifying the properties of starch. Such a product can be beneficial for creating a material with a lower viscosity and higher stability during processing15. Heat and shear stress are also improved16. The reduction of molecular weight can be achieved by oxidation with oxidants such as H2O217,, enzymatic or acidic hydrolysis of the main chain of the amylose and amylopectin16 or the cleavage of bonds by subjecting the native starch to heat7.
- Thermoplastic starch can be produced by subjecting native starch to casting. First thermoplastic starch is made into a water suspension heated between 130 – 150 ˚ At this temperature fragmentation and degradation occurs. At the top of the solution a film of starch forms, that can be used to be blown into articles. This process is mostly suitable for lab scale due to its non-continuous nature18.
- With thermal shear processing it is possible to turn native starch into a thermoplastic material. This process is conducted with the use of extruders that apply heat and shear to disrupt the crystallinity of the native starch into a continuous amorphous phase18. This shear also fragments the native starch chains. Often times this process requires the addition of plasticisers, such as glycerol or sorbitol, to improve chain mobility and improve flexibility and decrease hydrogen bonding. Other additives to improve mechanical properties, such as clays, are also added and homogeneously mixed during extrusion19. To improve mechanical performance and water resistance the native starch is in many cases blended with other polymers during extrusion, such as PHA7. The extrusion method is an easily up-scalable method.
Making decisions for the feedstock or processing methods can be based on different variables. The originating biomass for the starch feedstock is an important determining factor for the crystallinity and the crystallinity is the first and foremost barrier for processing. Making the “right” choice for feedstock makes all the difference. And while the degradation of the starch polymer chains occurs at lower temperatures, it’s not necessarily a bad thing. For the production of thermoplastic starch, the form of starch that is mostly used in starch articles, it’s even a necessity. The degree of fragmentation due to shearing during extrusion can be tuned with the type and amount of plasticizer. This also determines the viscosity and thus the possibility for certain processing techniques, such as injection moulding. For most of the common processing methods it’s possible to find compromises or formulations that are suitable. And indeed, several commercial products suitable for these processing techniques are available.
Biopar™ is a thermoplastic starch product of BIOP Biopolymer Technologies AG that is suitable for multiple processing methods. It can be extruded and used for film blowing or bottle blowing. It is suitable for the process of film casting, but also for injection moulding and thermoforming. The products made with the material mostly encompass packaging and bags19.
Biome Bioplastic is a British company that offers a thermoplastics starch that is suitable for use in Fused Deposition Modelling (additive manufacturing). The product allows for faster printing than commercial PLA products20.
Article by Wybren Kalsbeek
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