Thermoplastic starch is a promising material due to its biodegradability and sustainable source, but it’s naturally brittle and unsuitable for plastics because it lacks sub-Tg main-chain relaxation. Adding plasticizers like water and glycerol makes the starch molecules more flexible and can improve its properties. However, the water content significantly affects the thermal and mechanical properties of TPS, and the decline of material properties over time is a major drawback. Blending TPS with other polymers can enhance overall water resistance.
Plasticization of Starch
Thermoplastic starch has been garnering significant interest in research due to its biodegradability and bio-sustainable source. However, natural, dry starch has a relatively high glass transition temperature (Tg) and melting temperature (Tm) that make it brittle and unsuitable for use in plastics. This is because starch lacks a sub-Tg main-chain relaxation. Furthermore, the brittleness of starch increases over time due to free volume relaxation and retrogradation. To transform starch into a thermoplastic material, plasticization is required to make the starch molecules more flexible. This differs from other synthetic thermoplastic polymers that can be processed without plasticization.
Functional Modifications of Starch
The previous section covered the basic chemical modifications that can be done on starch to achieve desired properties. Starch can be modified with various functional groups depending on the desired outcome. However, studies have mainly focused on starch modified with long-chain fatty acid ester groups, which have shown promising results in improving flexibility and increasing elongation at break by reducing Tg and Tm. Currently, most thermoplastic starches are produced by adding low-molecular-weight plasticizers, such as water, glycerol, sorbitol, urea, amide, sugars, and quaternary amine.
Role of Water as a Plasticizer:
Thermoplastic starch benefits greatly from the presence of water as a plasticizer due to water’s strong attraction to the hydroxyl groups on starch. The amount of water required to prepare thermoplastic starch products is relatively low, typically less than 20% by weight during processing. Often, a small amount of glycerol is added to the mixture along with water to enhance the boiling point of water, which can slow down the migration and evaporation of water within the starch. An experiment was conducted to study the thermoplastic properties of potato starch with water and glycerol using extrusion. Results showed that samples containing less than 9% water by weight were glassy and had an elastic modulus of 400 MPa. The viscoelastic behavior of thermoplastic starch is highly dependent on the water content, typically exhibiting three regions: a glassy region when water content is lower than 9%, a glass-to-rubber transition followed by a rubbery region when water content is between 9% and 15%, and a viscous gel-like region with water content greater than 15%.
The effects of water as a plasticizer on the structure and properties of TPS were investigated. The extrudates of TPS with water exhibited translucency, suggesting the destruction of starch granular structure during extrusion. The native crystallinity of starch disappeared according to the X-ray diffraction results. Waxy maize starch displayed an A-type crystalline structure on the X-ray diffraction curve, which changed to amorphous morphology and a small amount of B-type crystallinity after being plasticized with water and extruded. Furthermore, thermal analysis indicated a significant decrease in the glass transition temperature (Tg) of the starch with increasing water content. The Tg of potato starch TPS dropped from 59°C with 5 wt% water content to -10°C with 20 wt% water content. In addition, research on starch/glycerol/water blends found that the Tg of TPS depends almost linearly on water content, with glycerol content fixed. Additionally, the amount of glycerol (above 14 wt%) had only a slight effect on Tg.
The thermal and mechanical properties of TPS are affected by the amount of water in the material, making it a major drawback of TPS-based materials. Over time, the water inside the starch can migrate and evaporate, or the starch may absorb water from the atmosphere, leading to a decline in material properties. Additionally, studies have shown that starch can post-crystallize in the presence of water and glycerol during storage, further exacerbating the decline in properties over time. The reorganization and crystallization of amylose and amylopectin molecules are partially responsible for the decline in material properties over time. This limitation of TPS makes it unsuitable for some applications that require high water resistance, but blending TPS with other polymers can help enhance overall water resistance.