Discriminating between Microsamples of Similar Resins with a Combination of FTIR and Thermal Analysis Instruments

Significance of the topic

The rapid, reliable identification of polymer microsamples is essential across quality control, failure analysis, environmental microplastic studies and contamination tracing. Combining surface-sensitive spectroscopic techniques with thermal analysis increases discriminatory power when polymers are compositionally similar or differ only by additives, copolymer content or thermal history. This approach enables analysts to characterize chemical composition, degradation state and inorganic additive load from sub-milligram samples.

Objectives and overview of the study

This study evaluated how a complementary workflow—ATR-FTIR, differential scanning calorimetry (DSC) and thermogravimetric/differential thermal analysis (TG-DTA)—can discriminate between six polypropylene (PP) microsamples (0.2–0.5 mg each). Goals were to: identify polymer type (homo- vs. copolymer), detect oxidative degradation, reveal thermal-history effects, and determine presence and approximate content of inorganic additives such as talc and glass fiber.

Used Instrumentation

The measurements employed three Shimadzu instruments:

• IRSpirit-TX FTIR spectrophotometer with QATR-S diamond ATR accessory for surface-sensitive IR spectra (4000–400 cm-1, 4 cm-1 resolution, 20 scans, SqrTriangle apodization)
• DSC-60 Plus differential scanning calorimeter (Al pan, 20 °C/min, 0→200 °C, N2 flow)
• DTG-60 simultaneous TG-DTA (Al pan, 20 °C/min, room temperature→500 °C, air flow)

Methodology

The workflow proceeded as follows:

• ATR-FTIR spectra were recorded for each intact PP particle to screen for polymer type, additive signatures and oxidative markers.
• The same microsamples (non-destructively) were subjected to DSC to probe melting behavior, enthalpy of fusion and effects of thermal history; repeated DSC heating was used to test reversible vs. history-related features.
• TG-DTA was used destructively to measure mass loss up to 500 °C and estimate inorganic residue fraction; residues remaining after TG-DTA were analyzed by ATR-FTIR to identify inorganic additives (talc, glass fiber).

Main results and discussion

FTIR findings:

• Sample 1 exhibited distinctive peaks (~1000, 680, 520 cm-1) consistent with talc and a carbonyl C=O stretch near 1730 cm-1 indicating oxidative degradation.
• Samples 3–6 showed a CH2 rocking band at ~720 cm-1, diagnostic of ethylene units (i.e., PP–ethylene copolymer); sample 2 lacked this band and was assigned as a PP homopolymer.
• Samples 1, 3 and 4 exhibited the carbonyl band at ~1730 cm-1, consistent with oxidative degradation events during manufacture or storage.

DSC observations:

• Sample 6 displayed a shoulder on the melting peak on first heating, suggesting the presence of multiple crystalline populations or thermal-history effects; upon reheating a single melting peak remained, indicating the shoulder was due to prior processing/thermal history rather than a compositional difference.
• Sample 4 showed a markedly lower heat of fusion relative to peers, suggesting reduced crystalline fraction or dilution by additives.

TG-DTA and residue analysis:

• TG showed that sample 6 lost nearly 100% of its mass up to 500 °C (no significant inorganic residue), while samples 3–5 retained measurable residues consistent with inorganic fillers.
• Residues were analyzed by ATR-FTIR: sample 3 residue matched glass fiber, residues of samples 4 and 5 contained both glass fiber and talc. These additives were sometimes undetected on intact-sample FTIR due to lack of surface exposure or low concentration.
• The larger residue and reduced heat of fusion for sample 4 are correlated and indicate a higher inorganic additive content diluting the polymer fraction.

Summary of sample assignment (key points):

• Sample 1: PP with significant talc content and oxidative degradation.
• Sample 2: Probable PP homopolymer (no 720 cm-1 band).
• Samples 3, 4, 5: Probable ethylene–propylene copolymers; glass fiber present (3), glass fiber + talc (4 and 5), with sample 4 having the highest inorganic load and evidence of oxidative degradation.
• Sample 6: Copolymer showing thermal-history effects (melting shoulder) but negligible inorganic residue.

Benefits and practical applications of the method

Combining ATR-FTIR with DSC and TG-DTA on the same microsample offers several practical benefits:

• High discrimination power for closely related resins—distinguishing homopolymer vs. copolymer, detecting oxidative degradation and revealing thermal history.
• Identification and semi-quantitative assessment of inorganic additives (talc, glass fiber) when coupled TG residue analysis with ATR-FTIR.
• Efficient use of very small samples (sub-milligram), enabling forensic, QC and microplastic analyses where sample mass is limited.
• Non-destructive FTIR screening allows targeted destructive thermal analyses only when needed, conserving scarce material.

Typical application areas include contamination source tracing in manufacturing, microplastic characterization in environmental studies, failure analysis of polymer parts and quality control of compounded resins.

Future trends and possibilities for use

Advances and likely directions to improve and expand this combined-analytics approach include:

• Hyphenated and microscale techniques: coupling micro-FTIR mapping, micro-DSC and microscale TG for spatially resolved composition/thermal profiling of single particles.
• Automated workflows and chemometrics: multivariate models trained on ATR-FTIR + thermal features to classify polymer grades, additive types and degradation levels with greater speed and objectivity.
• Improved residue analysis: integration with Raman spectroscopy or X-ray methods for more definitive inorganic identification and quantification of fillers.
• Expanded databases linking IR signatures, thermal transitions and decomposition profiles to commercial resin grades, enabling faster identification in industry and environmental monitoring.

Conclusion

A compact workflow combining ATR-FTIR, DSC and TG-DTA effectively discriminates among closely related polypropylene microsamples. Surface-sensitive FTIR rapidly identifies polymer class, specific additives exposed at the surface and oxidative damage. DSC reveals differences in crystallinity and thermal history that are not always apparent in FTIR spectra. TG-DTA provides mass-loss profiles and inorganic residue estimates; subsequent ATR-FTIR of residues confirms filler identity (talc, glass fiber). Using these complementary techniques on the same micro-sample allows robust characterization with minimal material consumption, useful for QA/QC, forensic analyses and microplastics research.

References

1. A. Kawaguchi, Discriminating between Microsamples of Similar Resins with a Combination of FTIR and Thermal Analysis Instruments, Shimadzu Application News, First Edition Mar. 2026, No. 01-01114-EN.
2. Shimadzu Application News No. 01-00710, Distinction of Polyethylene and Polypropylene by Infrared Spectrum.
3. Shimadzu Application News No. T159, Estimation of Thermal History of Polymer Using DSC-60 Plus Differential Scanning Calorimeter.