Starch is a natural polymer, a polysaccharide which functions as a carbohydrate (sugar) store, formed by photosynthesis when CO2 reacts with water. There are many possible sources, but the majority used in bio-based plastics comes from corn, wheat, cassava and potatoes. It is mixed with plasticiser to make thermoplastic starch (TPS), which can be formed using conventional moulding, extrusion and film-making processes.

Starch on its own cannot be used like plastic – it will breakdown rather than melt. Therefore, plasticisers are an important ingredient in the formation of workable material. They act as a bridge between the starch polymer chains, a link that can be melted and makes the material mouldable. Examples include glycerol, sorbitol and polyethylene glycol. In addition, TPS is blended with bio-based and biodegradable plastics to combine the benefits of both, such as improving toughness while maintaining a high rate of compostability. At around 0.5 usd/kg, starch is relatively low cost, helping make biodegradable plastics cheaper.

It is not durable in UV and will breakdown in most environments, including water, soil and composting conditions. Applications include things like packaging film and containers, food trays, beverage cups, flower pots and single-use bags (compost and refuse). Starch is used as an additive in biodegradable plastics to enhance their compostability. Whereas biodegradable polyesters, such as polylactic acid (PLA), may be certified compostable for up to 1 mm of thickness, PLA blended with starch may be compostable in thicker sections up to 1.5 mm. This greatly expands the possible range of applications to more rigid items, such as moulded packaging, take-away food containers and other single use items where petrochemical plastics are causing significant problems.

Beware of bioplastics made with a blend of starch and regular polymer, like polystyrene (PS). While these materials may have sufficient bio-based content to be classified as bioplastic, they are neither compostable nor biodegradable.