With the global implementation of plastic bans, the demand for biodegradable materials like PLA (Polylactic Acid) and PBAT (Polybutylene adipate terephthalate) has surged. However, these polymers are exceptionally sensitive to temperature and shear forces. During the compounding process, improper screw element configuration can easily trigger thermal degradation, resulting in decreased melt viscosity, yellowing of pellets, and a significant loss of mechanical properties.
Unlike traditional PE or PP, biodegradable plastics are prone to hydrolysis and thermal decomposition at elevated temperatures.
Narrow Processing Window: PLA is typically processed between 170°C and 190°C. If localized frictional heat (viscous dissipation) exceeds 200°C, the molecular weight drops rapidly.
Moisture Sensitivity: Even trace amounts of moisture can cause ester bond cleavage under high pressure. Therefore, degassing efficiency (venting) is a decisive factor for final product quality.
In the selection and configuration of a twin screw extruder for PLA/PBAT, the guiding principles are "Low Temperature, Low Shear, and High Distribution."
Avoid Excessive Shear: Reduce the use of 90-degree neutral kneading blocks. Instead, increase 30-degree or 45-degree forward-conveying kneading blocks to ensure mixing occurs while the material is moving, minimizing heat buildup from long residence times.
Distributive Mixing Elements: We recommend incorporating SME (Screw Mixing Element) or TME (Turbine Mixing Element) elements. These components effectively disperse additives (such as starch or CaCO3) without generating excessive frictional heat.
Technical Parameter: The unilateral clearance between the screw elements and the barrel wall should be maintained between 0.02 mm and 0.05 mm.
Objective: Tight clearance prevents material backflow and prevents localized overheating, ensuring stable pressure output and consistent melt quality. (Reference: Factory Assembly Precision Report - Ref: #INSP-2023-V3)
Since biodegradable compounding often involves starch or other natural fillers, the screw and barrel must possess sufficient durability:
Barrel Material: A bimetallic barrel lined with high-chrome alloy is recommended to resist abrasion from natural fillers.
Hardness Standard: Screw elements should achieve a hardness of 58-64 HRC via vacuum quenching to ensure longevity and prevent metal contamination in the final product.
For manufacturers of PLA/PBAT compounds, the precision of extruder parts determines the success of degradation control. By optimizing screw geometries, maintaining temperature stability within +/- 1°C, and utilizing high-compatibility screw elements for Coperion or Leistritz models, companies can maximize throughput while preserving the material's essential mechanical integrity.
