Among the oldest and most affordable bioplastics. Starch, from corn, potato, wheat or cassava, is abundant and cheap, but pure starch makes a poor plastic, so it's almost always blended with other polymers to become genuinely useful.
| Property | Pure TPS | Starch–PBAT / PLA blend |
|---|---|---|
| Tensile strength | 2–6 MPa | 10–25 MPa |
| Elongation at break | high but weak | tunable, film-grade |
| Water sensitivity | very high | much reduced by blend partner |
| Density | ~1.3–1.4 g/cm³ | ~1.3 g/cm³ |
| Cost | lowest of all bioplastics | low–moderate |
| Home compostable | often yes | depends on blend, verify |
Starch is hygroscopic, it absorbs water from the air, and its mechanical properties shift with moisture content and with the amount of plasticizer (glycerol) used. That's why a starch material's strength and flexibility are best described as a range that the formulator dials in, not a fixed spec. It's also why the blend partner matters so much: it stabilises properties that pure starch lets drift.
Starch is the energy store of plants, the same stuff in a potato or a corn kernel. It's one of the most abundant and cheapest biopolymers on Earth, which is exactly why it's attractive as a plastic feedstock. The best-known commercial starch bioplastic is the Mater-Bi family, which built a global compostables business on starch chemistry.
Native starch granules aren't a plastic. To make them one, starch is "plasticized", heated with water and a plasticizer such as glycerol until the granules break down into a continuous, mouldable material called thermoplastic starch (TPS). On its own, TPS is weak and absorbs water, so it's then compounded with tougher, water-resistant biopolymers, most often PBAT or PLAto create a blend with usable mechanical properties.
Almost no commercial starch bioplastic is pure starch. The starch keeps it cheap and compostable; the blend partner makes it strong and moisture-tolerant. When you see a compostable carrier bag, it's very often a starch-PBAT blend.
Economics and end-of-life. Starch is cheap and renewable, and starch blends compost readily, many are certified for industrial and even home composting. That combination makes them the default choice for compostable bags, films, and loose-fill packaging where cost matters and the disposal route is composting rather than recycling.
Water is starch's weakness. Starch loves moisture, which is great for composting but bad for, say, a drink container, humidity and contact with water degrade its properties. Pure starch is also mechanically weak, which is why blends dominate. And properties can drift over time as the plasticizer migrates. Starch blends are excellent where the job is "hold something dry, then compost", and poorly suited to wet or load-bearing uses.
This is where starch blends shine: they are among the more reliably compostable bioplastics, with many products certified for home as well as industrial composting (always check the specific certification, the blend partner matters). In the right disposal system they return cleanly to the soil.
Related: PBAT (the usual blend partner) · PLA · Compostability standards
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