The optimal ratio of starch to glycerol for preparing biodegradable films can vary based on the specific type of starch, the desired film properties, and the method of preparation. However, a commonly used ratio for achieving a good balance between mechanical properties and water resistance is typically around 1:0.3 to 1:0.5 (starch:glycerol by weight).

1. **Starch**: Serves as the primary structural component, providing rigidity and strength to the film.

2. **Glycerol**: Acts as a plasticizer, enhancing flexibility and reducing brittleness, which improves the film's mechanical properties.

In general:

- **Higher glycerol content** (e.g., 1:0.5) can improve flexibility and reduce brittleness, but may decrease tensile strength and increase water permeability.

- **Lower glycerol content** (e.g., 1:0.3) can enhance tensile strength and reduce water permeability, but may lead to a more brittle film.

It is crucial to conduct experimental trials to determine the optimal ratio for your specific application, as other factors such as the type of starch (e.g., corn, potato, tapioca), the presence of additives, and the processing conditions (e.g., casting, extrusion) can significantly influence the properties of the final film. Additionally, incorporating additives like plasticizers or other natural polymers may further enhance the properties of the biodegradable films.

Here I make some additional remarks on the valuable comments of Şeyda Bostancı. The first basic problem with thermoplastic starch (TPS) is that it is stable in time only when stored at a temperature below the glass transition temperature Tg (like any other amorphous material) but that at these conditions the material is brittle, i.e. it has a low elongation at break. The second problem is that TPS is sensitive to water and that it tends to crystallise. Because of the poor mechanical properties of TPS with high Tg, the desired Tg has to be somewhat lower than T at storage and use.

At a starch concentration of 70 % and higher, the optimum crystallisation temperature is > 50 ˚C (De Meuter, attached). This suggests that crystallisation during storage is not necessarily a problem if storage and use are at temperate (i.e. not tropical) conditions.

The equilibrium water content of starch is (very roughly) 15 % (wet basis) = 17.6 % (dry basis) at about 65 % relative humidity. Hence, for TPS with 70 % starch the optimal water content (where no big changes during storage are expected) is approximately 12.3 % (of total TPS). Of course, if water activity in the application changes a lot, e.g. when used in packaging of wet or (very dry) materials, water uptake or release will occur.

Lately, the use of other plasticisers for enhancing the functional properties of TPS have been discussed, alone or combined with glycerol. These include sorbitol, ethanolamine, and citric acid. Some abstracts and papers are attached.

Sorry, I forgot one attachment!