Thermal Degradation of Polylactic Acid During Processing: Key Factors and Mitigation Strategies

Abstract

Polylactic acid (PLA) is widely recognized as one of the most promising bio-based and biodegradable polymers. Its renewable origin, compostability, and favorable mechanical and optical properties have enabled broad applications in fibers, films, packaging, and consumer products. However, PLA exhibits relatively poor thermal stability. During melt processing—especially in fiber spinning, film extrusion, and thin-layer coating—thermal degradation can be triggered when the melt temperature or residence time becomes excessive. Degradation leads to chain scission, molecular weight reduction, discoloration, and poor mechanical performance. This article provides a technical overview of the major factors influencing PLA thermal degradation and outlines practical engineering strategies to minimize degradation during industrial processing.

1. Introduction

As the global demand for environmentally friendly materials continues to rise, PLA has emerged as the flagship biodegradable polymer for large-scale applications. PLA is produced from renewable feedstocks such as corn starch or sugarcane, and its end-of-life options include industrial composting and chemical recycling. Despite these advantages, PLA remains highly sensitive to thermal and hydrolytic degradation because of its aliphatic polyester backbone containing easily cleaved ester groups.

During melt processing, PLA must be heated above 170–180 °C to achieve flowability. In some applications—such as microfiber spinning or ultra-thin film coating—the melt temperature may approach 220–230 °C to meet flow requirements. This elevates the risk of random chain scission, oxidative degradation, and hydrolysis. Therefore, understanding the degradation mechanisms and the influence of processing parameters is vital for ensuring product quality and long-term performance.

2. Key Factors Affecting Thermal Degradation During PLA Processing

2.1 Processing Temperature

Temperature is the most dominant factor governing thermal degradation.

• PLA melts at 150–170 °C, but typical processing temperatures are 180–220 °C.

• When the temperature exceeds 230 °C, degradation accelerates sharply, producing lactide, acetaldehyde, carbon monoxide, and oligomers.

• Local overheating, often caused by frictional heating or poor temperature uniformity, can push hotspots above 240 °C, resulting in severe discoloration and molecular weight loss.

Studies have shown that at 230–240 °C, PLA’s molecular weight can decrease by more than 30% within minutes (Lehermeier & Dorgan, 2002).

2.2 Residence Time

Prolonged exposure to high temperature intensifies degradation.

• In extrusion, low screw speeds or unbalanced feeding increase residence time.

• A residence time longer than 3 minutes at high temperature typically results in measurable chain scission.

• For complex molds or thick parts, slow cooling may allow degradation to continue even after the melt leaves the extruder.

Residence time control is therefore just as important as temperature control.

2.3 Moisture Content in Raw Materials

PLA is hygroscopic and readily absorbs moisture from the environment.

• Moisture triggers hydrolytic degradation, causing rapid molecular weight reduction under high temperature.

• When moisture content exceeds 0.05%, hydrolysis becomes significant.

• Hydrolysis also produces gases that create bubbles, pinholes, and weak points in fibers and films.

Drying PLA to < 0.02% moisture is necessary for stable processing.

2.4 Shear Stress and Mechanical Forces

High shear rates can cause mechanical chain scission and localized temperature spikes:

• Screen packs, mixing elements, and barrier screws increase shear intensity.

• When the shear rate exceeds 1000 s⁻¹, degradation accelerates due to mechanical stress and localized heating (“shear burning”).

• High-shear screws must be used cautiously in PLA systems.

2.5 Impurities and Catalyst Residues

Residual catalysts, metal ions, or incompatible additives can catalyze PLA degradation.

• Tin-based catalysts from PLA polymerization are known to accelerate degradation.

• Certain plasticizers and lubricants may induce chain scission if poorly compatible.

Using high-purity grades and screened additives is essential.

3. Strategies to Mitigate PLA Thermal Degradation

3.1 Raw Material Pretreatment

• Drying: 80–100 °C for 4–6 hours (vacuum drying recommended) to achieve moisture <0.02%.

• Material selection: Choose high-purity PLA grades with low catalyst residue.

3.2 Optimized Processing Conditions

• Maintain melt temperature below 210–220 °C whenever possible.

• Implement zoned temperature control to avoid local overheating.

• Increase screw speed moderately to reduce residence time, but avoid excessive shear.

• Residence time should be ≤3 minutes for stable molecular weight retention.

3.3 Equipment and Screw Design

• Use low-shear or moderate-compression screws.

• Avoid sharp turns or abrupt changes in flow channels.

• Install vacuum venting in extruders to remove volatile degradation products (e.g., acetaldehyde).

3.4 Use of Stabilizers

Thermal stabilizers can act as a chemical “protection shield.”

• Hindered phenol antioxidants (e.g., AO-1010)

• Phosphite stabilizers (e.g., Irgafos 168)

• UV absorbers for light-sensitive applications

Stabilizers must be used in small amounts (0.1–0.3%) to avoid compromising compostability.

 

4. Conclusion

PLA’s susceptibility to thermal degradation poses challenges in high-temperature melt processing. However, by controlling temperature, residence time, moisture levels, shear forces, and impurities, manufacturers can significantly improve PLA’s processing stability and product quality. These mitigation strategies enable PLA to fulfill demanding industrial applications while maintaining its environmental advantages.

As global interest in biodegradable materials grows, optimizing PLA’s processing stability will be critical for scaling its use across textiles, films, consumer products, and beyond.

 

5. References

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Jamshidian, M., et al. (2010). Comprehensive Reviews in Food Science and Food Safety, 9(5), 552–571.

Lehermeier, H. J., & Dorgan, J. R. (2002). Journal of Rheology, 46(4), 747–777.

Liu, H., & Zhang, J. (2011). Journal of Polymer Science Part B, 49(12), 1051–1083.

Drieskens, M., et al. (2009). Polymer Testing, 28(7), 729–735.