Multifaceted Evaluation of Plastics: Difference of Heat Treatment Conditions

Significance of the Topic

Polylactic acid (PLA) is a plant derived polymer that offers reduced environmental impact compared to petroleum based plastics but typically exhibits lower heat resistance and mechanical durability. Annealing has been applied to improve these properties by inducing crystallization. Understanding the relationship between annealing conditions and material behavior is essential for optimizing performance in industrial applications.

Objectives and Study Overview

This study investigated the effects of annealing treatment at 100 °C for 30 minutes on PLA. A multifaceted evaluation approach was adopted combining tensile testing, micro hardness measurements, differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The goal was to correlate mechanical performance changes with crystallization behavior.

Methodology

Tensile tests were conducted using a precision universal tester at 1 mm per minute with five replicates. Hardness was measured by a DUH 210 micro hardness tester under a 500 mN Berkovich indenter. DSC analysis employed a 10 °C per minute heating rate under nitrogen flow. FTIR spectra were recorded with a diamond ATR accessory over 20 scans and 4 cm-1 resolution.

Instrumentation Used

• AGX V Precision Universal Tester
• DUH 210 Dynamic Ultra Micro Hardness Tester
• DSC 60 Plus Differential Scanning Calorimeter
• IRTracer 100 FTIR Spectrophotometer with QATR 10 accessory

Main Results and Discussion

Annealing increased tensile strength and elastic modulus while reducing elongation at break by approximately half. Hardness HIT rose from about 222 to 278 MPa. DSC revealed that unannealed PLA exhibited a glass transition at 56 °C and a cold crystallization peak at 114 °C, both absent after annealing, confirming prior crystallization. FTIR analysis indicated a decrease in the amorphous band at 955 cm-1 and an increase in the α crystalline band at 921 cm-1, further supporting enhanced crystallinity.

Benefits and Practical Applications of the Method

The combined use of mechanical, thermal and spectroscopic techniques allows a clear linkage between processing conditions and material structure. Hardness testing on actual part geometries provides a minimally destructive method for quality control and inline evaluation. Accumulated data from these tests can support rapid assessment of production batches.

Future Trends and Opportunities

Future work may focus on establishing quantitative correlations between hardness values and tensile parameters to enable non destructive evaluation of parts. Extending this approach to other biopolymers and integrating inline thermal and spectroscopic monitoring can enhance process control and reduce waste. Automation of data analysis with machine learning could further accelerate material optimization.

Conclusion

Annealing at 100 °C for 30 minutes significantly enhances the mechanical and thermal performance of PLA through induced crystallization. A multifaceted evaluation strategy proves effective for understanding these changes and offers practical benefits for quality assurance and manufacturing optimization.

References

(1) J M Zhang Y X Duan H Sato H Tsuji I Noda S Yan and Y Ozaki Macromolecules 2005 38 8012