Compostable carrier bags, produce bags and mulch film are rarely made from a single biodegradable polymer resin. Most are based on blends of PBAT and PLA, often with fillers. This article explains why the two resins are combined, what each one contributes to the film, and what to ask for when you source them.
Two resins with opposite strengths
PLA (polylactic acid) is produced from renewable feedstocks such as corn or sugarcane. It is stiff, strong and clear, with a glass transition temperature of roughly 55 to 60 °C. On its own, PLA film is too brittle and noisy for most bag and wrap applications.
PBAT (polybutylene adipate terephthalate) is a flexible, aliphatic-aromatic copolyester. It behaves much like LDPE in film: soft, tough and highly extensible, with good tear and puncture resistance. Its weaknesses are low stiffness and relatively high cost per metric ton of film.
Blending the two gives a film that is tougher than PLA and stiffer than PBAT. PBAT usually forms the larger share in flexible film, with PLA added to raise stiffness, reduce stretch and improve printability.
Making the blend work
PLA and PBAT are not fully miscible, so the way the blend is compounded has a large effect on film quality. Compound producers use compatibilizers or chain extenders to improve the interface between the two phases and to stabilize melt strength for blown film. Mineral fillers such as calcium carbonate, or starch, are sometimes added to adjust stiffness and cost.
For a converter, the practical choice is usually between purchasing a ready-to-use compounded blend, or purchasing PBAT and PLA separately and blending in-house. Ready-made compounds are simpler to run and give more consistent film. In-house blending gives more control over the recipe, but needs suitable compounding equipment and experience.
Processing points
- Drying: both PLA and PBAT are polyesters and can lose molecular weight through hydrolysis if processed wet. Keep bags sealed and dry the resin as recommended by the producer.
- Temperature: these materials run at lower melt temperatures than many conventional polyolefins. Overheating reduces melt strength and can cause bubble instability.
- Storage: store resin and finished film in cool, dry conditions. Biodegradable film has a shorter shelf life than PE film, so plan stock rotation.
Compostability and certification
A film is not compostable just because its resins are. Certification standards such as EN 13432 in Europe and ASTM D6400 in the United States apply to the final product, including thickness, inks and additives. When you source resin for certified products, ask which certificates cover the specific grade or compound, and whether they cover industrial composting, home composting, or both.
What to ask your supplier
- Is the grade pure PBAT, pure PLA, or a compounded blend? If a blend, which application is it designed for?
- MFR, density, and the recommended processing and drying conditions.
- Copies of the TDS, MSDS and the relevant compostability certificates for the grade.
- Packing, moisture protection during shipping, and recommended storage time.
Full Trust supplies PBAT resin, PLA resin and other biodegradable materials. Tell us your film application and thickness, and we will suggest grades and share their documents.
