Ep 24 — Polymer Industry: Degradation and Aging Assessment

Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Part: Part III · Intermediate — Industry Applications
Audience: Polymer aging researchers, outdoor product engineers, failure analysis engineers
Prerequisites: Ep 23 (Plastic Identification), Ep 14 (ATR), Ep 18 (Spectrum Processing)
Reading Time: Approx. 45 minutes

Introduction: A "Five-Year-Old" Greenhouse Film

In 2023, an agricultural cooperative in Xinjiang reported that greenhouse PE film cracked extensively after only 18 months of use, far below the manufacturer's promised "5-year service life." A sample was sent to the lab, and a technician performed ATR-FTIR scanning — the results were startling: a strong carbonyl absorption peak at 1715 cm⁻¹ appeared on the film surface, with intensity even exceeding the original PE CH₂ peak at 1463 cm⁻¹ [1].

The Carbonyl Index (CI) was calculated to be as high as 1.2, while the CI of a normal 5-year-old PE film is typically ≤ 0.3. This indicated severe photo-oxidative aging far beyond the normal aging rate [1][2].

Further investigation revealed that the batch of PE film had insufficient addition of light stabilizer (HALS), combined with strong local UV and high temperatures, leading to accelerated aging. Ultimately, the manufacturer replaced all the film for the cooperative, incurring losses of millions of yuan.

"FTIR carbonyl index is the single most useful parameter for assessing the photo-oxidative degradation of polyolefins."
—— Gardette J L et al., Polymer Degradation and Stability, 2013 [2]

In this episode, we will delve into the world of polymer degradation and aging using infrared spectroscopy — understanding aging mechanisms, mastering carbonyl index measurement methods, distinguishing different aging modes, and conducting a practical assessment through a 5-year aging case of outdoor PE film.

1. Why Do Polymers "Age"?

1.1 The Essence of Aging: Molecular Structure Changes

During use, polymers are affected by light, heat, oxygen, water, mechanical stress and other factors, causing irreversible changes such as chain scission, crosslinking, oxidation in the molecular chain, leading to performance degradation [2][3]:

1.2 Effects of Aging on Polymer Properties

1.3 Why FTIR is the First Choice for Aging Assessment?

The essence of aging is chemical bond changes — and infrared spectroscopy is precisely the "eye to see chemical bonds" [2][3]:

• Sensitive: Oxygen-containing groups generated by aging (carbonyl, hydroxyl, etc.) have strong absorption in the infrared region
• Quantitative: Characteristic peak area can be used to measure aging degree
• Non-destructive: Retain samples for mechanical testing
• Micro-area: μ-FTIR can analyze aging gradient distribution
• ATR surface: Surface aging (most aging starts from the surface) is naturally suited for ATR

2. Photo-oxidation Mechanism and Infrared Characteristics

2.1 Polyolefin Photo-oxidation Mechanism

Photo-oxidation of polyolefins such as PE and PP is a typical free radical chain reaction [2][4]:

① Initiation

UV light excites chromophores in the polymer — possibly impurities, catalyst residues, thermally oxidized carbonyl groups, etc.:

$$\text{RH} \xrightarrow{h\nu} \text{R}^\bullet + \text{H}^\bullet$$

② Propagation

Alkyl radicals react with O₂ to form peroxy radicals, which then abstract hydrogen to form hydroperoxides:

$$\text{R}^\bullet + \text{O}_2 \rightarrow \text{ROO}^\bullet$$

$$\text{ROO}^\bullet + \text{RH} \rightarrow \text{ROOH} + \text{R}^\bullet$$

③ Chain Branching

Hydroperoxides decompose under UV or heat, generating more radicals:

$$\text{ROOH} \xrightarrow{h\nu} \text{RO}^\bullet + {}^\bullet\text{OH}$$

④ Product Formation

Radical recombination produces various oxygen-containing groups [2][4]:

Figure 1: Main products and infrared characteristic peaks of polyolefin photo-oxidation

2.2 "Fingerprints" of Infrared Characteristic Products

Comparing the aged PE spectrum with the original, the most significant changes are [1][2]:

① Strong broad peak in carbonyl region 1650–1780 cm⁻¹

• Superposition of various carbonyl products
• Center around 1715 cm⁻¹ (mainly ketones)
• Most intuitive indicator of aging

② Broad peak in O-H region 3200–3600 cm⁻¹

• O-H of hydroperoxides, alcohols, acids
• Unlike the weak O-H of original PE, it broadens and strengthens after aging

③ Enhancement in C-O-C region 1000–1300 cm⁻¹

• Esters, ethers, and other oxygen-containing groups

④ Trans double bond at 965 cm⁻¹

• Vinyl groups produced by chain scission
• Appears in some samples after aging

🔗 Extension: The carbonyl produced by photo-oxidation is precisely the typical absorption position of "carbonyl" in the ftir.fun database. On the ftir.fun carbonyl functional group page, 1700 and 1740 cm⁻¹ are two common carbonyl frequencies — the former corresponds to ketones/carboxylic acids, and the latter to esters/aldehydes [5].

📷 Figure 2: FTIR spectral changes during PE photo-oxidation at different times
Source: Gardette J L et al., Polymer Degradation and Stability, 2013 [2]
https://doi.org/10.1016/j.pol…

3. Carbonyl Index

3.1 Definition

Carbonyl Index (CI) is a standardized indicator for measuring the oxidation degree of polymers [1][2][6]:

$$CI = \frac{A_{C=O}}{A_{ref}}$$

where:

• $A_{C=O}$: Absorbance or peak area of carbonyl peak (typically 1715 cm⁻¹)
• $A_{ref}$: Absorbance or peak area of reference peak

3.2 Selection of Reference Peak

Different literature uses different reference peaks; consistency must be ensured [1][6]:

Note: When selecting a reference peak, ensure it is unaffected by aging. Commonly used reference peaks include:

• PE: 1463 cm⁻¹ (CH₂ bending) or 718 cm⁻¹ (CH₂ rocking)
• PP: 1167 cm⁻¹ (C-C stretching)
• PET: 1410 cm⁻¹ (C-H bending) or 720 cm⁻¹ (C-H rocking)
• Using internal standard peak area ratio can reduce spectrum normalization errors

4. Case Study: 5-Year Outdoor PE Film Aging Assessment

|--------|------------|------|------|
| CH₂ scissoring | 1463 | Strong and stable, common in PE | Varies with different matrices |
| CH₂ antisymmetric stretching | 2920 | Strong, clearly distinguishable from C=O | Small change after aging |
| CH₂ rocking | 720 | Characteristic of PE | Affected by crystallinity |
| Maximum spectral peak | Varies | Universal | Unstable |

Recommendation: PE use 1463 cm⁻¹, PP use 1376 cm⁻¹, PET use 1410 cm⁻¹ [1][6].

3.3 Peak Area vs Peak Height

• Peak Area (recommended): Integrate the 1650–1780 cm⁻¹ region, insensitive to baseline tilt
• Peak Height: Measure absorbance at 1715 cm⁻¹, simple but susceptible to baseline effects

Calculation formula (peak area method) [1]:

$$CI = \frac{\int_{1780}^{1650} A(\nu) \, d\nu}{\int_{\nu_2}^{\nu_1} A(\nu) \, d\nu}$$

3.4 Physical Meaning of Carbonyl Index

CI is a relative value and must be compared with unaged samples under the same conditions [1][6]:

⚠️ Note: CI values from different matrices (PE, PP, PS) cannot be directly compared — reference peak intensities differ. Comparisons are meaningful only within the same material.

3.5 Example: PE Aging Curve

CI variation for a PE film exposed outdoors for different durations [1]:

   CI
   1.2 ┤                          ●
   1.0 ┤                      ●
   0.8 ┤                  ●
   0.6 ┤              ●
   0.4 ┤          ●
   0.2 ┤      ●
   0.0 ┤  ●
       └─────────────────────────
        0   6  12 18 24 30 36 months

        Outdoor exposure time →

Figure 3: CI vs. time curve for PE film exposed outdoors (schematic)

Key observations:

• Initially, CI increases slowly (induction period)
• Mid-stage, CI accelerates (catalytic degradation)
• Later, it plateaus (complete aging)

IV. Infrared Differences Between Thermal Oxidation and Photooxidation

4.1 Mechanism of Thermal Oxidation

Thermal and photooxidation share similar free radical mechanisms, but differ in initiation steps [3][4]:

• Photooxidation: Initiated by UV excitation of chromophores
• Thermal oxidation: Initiated by thermal energy providing activation energy

Thermal oxidation can occur without light (e.g., underground pipes, electrical interiors), at a slower but sustained rate.

4.2 Infrared Spectral Differences

Photooxidation and thermal oxidation show subtle differences in IR spectra [2][3]:

Reasons:

• Photooxidation produces more chain scission, forming ketones and vinyl groups
• Thermal oxidation favors crosslinking, forming more esters and ethers

4.3 Case: Distinguishing Photooxidation vs Thermal Oxidation

A PP part failed after 3 years of outdoor use. Determine whether photooxidation or thermal oxidation is responsible [3]:

• Strong 1715 cm⁻¹ → severe oxidation
• Weak peak at 965 cm⁻¹ → indication of photooxidation
• Peak at 1640 cm⁻¹ → further supports photooxidation
• Ratio 1735 cm⁻¹ / 1715 cm⁻¹ = 0.4 → low ester/ketone ratio (photooxidation characteristic)

Conclusion: Primarily photooxidation. Check HALS light stabilizer content.

V. Hydrolysis Aging Monitoring

5.1 Hydrolysis Mechanism

Polymers containing ester bonds or amide bonds can hydrolyze in the presence of water [3][7]:

Polyester hydrolysis (PET, PC):
$$\text{-CO-O-} + \text{H}_2\text{O} \rightarrow \text{-COOH} + \text{-OH}$$

Polyamide hydrolysis (PA):
$$\text{-CO-NH-} + \text{H}_2\text{O} \rightarrow \text{-COOH} + \text{-NH}_2$$

Polyurethane hydrolysis (PU, using urethane bond as example):
$$\text{-O-CO-NH-} + \text{H}_2\text{O} \rightarrow \text{-OH} + \text{CO}_2 + \text{-NH}_2\quad(\text{simplified schematic})$$
A more complete pathway often involves carbamate bond cleavage generating amines and alcohols/carboxylic acids; if the soft segment is polyester, ester bond hydrolysis also occurs, typically seen in IR as decrease of ester carbonyl and increase of carboxylic acid carbonyl, rather than a general "enhancement at 1700 cm⁻¹".

5.2 Infrared Monitoring of Hydrolysis

5.2.1 Polyurethane (PU) Hydrolysis [7]

Typical IR changes in PU hydrolysis:

• 3300 cm⁻¹ (N-H): Strengthens and broadens due to NH₂ formation
• 1700–1730 cm⁻¹ (C=O): Peak shape changes due to ester bond cleavage and acid formation
• 1530 cm⁻¹ (amide II): Enhances due to increased NH₂
• 1700 cm⁻¹ (carboxylic acid C=O): New peak appears

🔗 Extension: For PU-related C=O region, refer to ftir.fun carbonyl functional group page (the peak statistics on the page will update as literature is added; do not treat a single 'support count' as constant). When determining hydrolysis, consider polymer type: polyester-based soft segment PU pays more attention to the increase/decrease of ester/carboxylic carbonyl.

5.2.2 Polyamide (PA) Hydrolysis

After PA hydrolysis [3]:

• 3300 cm⁻¹ (N-H): Little change (primary amine formation)
• 1635 cm⁻¹ (amide I): Slightly weaker
• 1540 cm⁻¹ (amide II): Slightly weaker
• 1710 cm⁻¹ (carboxylic acid C=O): New peak appears

Key determination: The appearance of a carboxylic acid peak at 1710 cm⁻¹ is the most direct evidence of hydrolysis.

🔗 Extension: Amide I (1635 cm⁻¹) and amide II (1540 cm⁻¹) in PA are typical amide double peaks. On the ftir.fun amide functional group page, 1650 and 1550 cm⁻¹ are common amide peak positions [8]. PA hydrolysis weakens these two peaks and enhances the peak at 1710 cm⁻¹.

5.3 Quantification of Hydrolysis Degree

Hydrolysis degree can be quantified by the change in amide II peak area [3][7]:

$$\text{Hydrolysis degree} = 1 - \frac{A_{1540, t}}{A_{1540, 0}}$$

where $A{1540, t}$ is the amide II peak area after hydrolysis time $t$, and $A{1540, 0}$ is the initial peak area.

VI. Case Study: 5-Year Outdoor Aging Assessment of PE Film

6.1 Case Background

A PE agricultural greenhouse film, with a nominal service life of 5 years. After 5 years of use, evaluate whether it can continue to be used and analyze the aging pattern [1].

6.2 Assessment Method

① Sample Collection

• Take samples from different parts of the film (light-facing side, back-light side, edge in contact with soil)
• Each sample 5×5 cm
• Also take unused film from the same batch as control

② ATR-FTIR Measurement

• Diamond ATR, 4 cm⁻¹, 32 scans
• Measure 3 different points per sample

③ Mechanical Property Testing

• Tensile strength, elongation at break (GB/T 1040)

④ Carbonyl Index Calculation

• $CI = A{1715} / A{1463}$ (peak area method)

6.3 Evaluation Results

Table 1: Aging evaluation data for different parts of PE film

6.4 Data Interpretation

① Correlation between CI and Mechanical Properties

Higher CI, lower tensile strength and elongation at break—consistent with molecular chain scission and performance degradation due to aging [1][2].

② Light-facing side vs. Back side

CI on the light-facing side is significantly higher than on the back side, showing a clear aging gradient—a typical feature of outdoor plastic aging, with light as the main cause [2].

③ Special aging at the soil side

CI at the soil side is higher than at the back side but lower than at the light-facing side—possibly due to synergistic aging from soil moisture + microorganisms + chemicals [1].

6.5 Judgment and Recommendations

• Light-facing side: CI=0.42, elongation at break 220%—critical
• Overall aging is evident, replacement recommended
• Aging is uneven across different parts; pay attention to inspection of the light-facing side during use

6.6 Further Analysis of the O-H Region

Besides the carbonyl index, changes in the O-H region also have diagnostic value:

• Control sample: almost no peak at 3400 cm⁻¹
• Aged sample: a broad peak at 3400 cm⁻¹, corresponding to O-H from hydroperoxides, alcohols, acids

🔗 Extension: The O-H produced by aging is exactly the typical location of "hydroxyl" in the ftir.fun database. On the ftir.fun hydroxyl functional group page, 3400 and 3300 cm⁻¹ are common O-H peak positions [9]. The broad peak at 3400 cm⁻¹ in aged PE is a manifestation of this functional group.

VII. Other Aging Indices and Comprehensive Evaluation

7.1 Aging Indices Besides CI

7.2 Principles for Selecting Aging Indices [1][6]

① Polyolefins (PE, PP)

• Prefer CI (most sensitive)
• HI as auxiliary

② Polyesters (PET, PC)

• Use the 1710 cm⁻¹ carboxylic acid peak
• HI (3400 cm⁻¹) to monitor hydrolysis

③ Polyamides (PA)

• Use changes in amide II (1540 cm⁻¹)
• Monitor the 1710 cm⁻¹ carboxylic acid peak

④ Polyurethanes (PU)

• Use the ratio of 1700 vs 1730 cm⁻¹
• HI to monitor hydrolysis

7.3 Standardization of Test Conditions

Comparison of aging indices requires consistent test conditions [1][6]:

① ATR Measurement Conditions

• Same crystal, same incidence angle
• Same pressure (e.g., 30 N)
• Same number of scans (≥32)

② Sample State

• Clean surface (no dust, oil)
• Dry (no adsorbed water)
• Flat surface

③ Data Processing

• Same baseline correction method
• Same integration range
• Same reference peak

⚠️ Key point: Aging indices like CI are relative values; CIs obtained from different labs, instruments, or methods cannot be directly compared—only trend comparisons using the same method are valid [1].

VIII. Industry Frontiers and Developments

8.1 Chemical Imaging of Aging Processes

Using FPA-ATR imaging can observe the spatial distribution of aging [10]:

• Micro-scale resolution (~5–10 μm)
• Can see the "gradient" of aging penetrating from the surface inward
• Can identify local aging hot spots (e.g., stress concentration points)

8.2 Machine Learning for Aging Prediction

Machine learning models based on large amounts of aging data [10]:

• Input: FTIR spectra + exposure time
• Output: Remaining life prediction
• Can be used for "health management" of outdoor products

8.3 Terahertz Spectroscopy

Terahertz spectroscopy is sensitive to changes in polymer crystallinity:

• Secondary crystallization caused by aging
• Complementary to FTIR

IX. Industry Experience and "Pitfall" Records

9.1 Pitfall 1: Inconsistent CI Calculation Methods

Case [1]: Two lab reports on the same PE sample gave CIs of 0.35 and 0.55—huge discrepancy.

Reasons:

• Lab A used peak height method, Lab B used peak area method
• Lab A reference peak 1463, Lab B reference peak 2920
• Different methods, cannot be directly compared

Lesson:

• CI reports must specify the method (peak height/area, reference peak, integration range)
• Comparison requires confirming method consistency

9.2 Pitfall 2: Misjudgment Due to Surface Contamination

Case [1]: A PE film yielded CI=0.8, seemingly severe aging. Inspection revealed surface soil contamination—ATR measured the contaminant, not the plastic itself.

Lesson:

• Clean sample surface before ATR measurement
• Wipe with ethanol or cut a fresh cross-section
• Rinse with water and dry if necessary

9.3 Pitfall 3: Ignoring ATR Penetration Depth

Case [1]: A thick PC plate showed high CI, but it was only surface aging. After removing 50 μm from the surface, CI dropped significantly.

Reason: ATR penetration depth is only 1–2 μm, only sees the surface—surface aging is often more severe than the bulk.

Lesson:

• ATR CI reflects surface aging degree
• For bulk aging, slice for transmission measurement
• Or cut a cross-section with a diamond knife for ATR

9.4 Pitfall 4: Ignoring Effect of Light Stabilizers

Case [2]: A PE film containing HALS (hindered amine light stabilizer) showed lower-than-expected CI—the N-H peak of HALS (3300 cm⁻¹) overlaps with O-H, interfering with HI calculation.

Lesson:

• Consider additive interference when calculating aging indices
• HALS N-H at 3300 cm⁻¹ interferes with HI
• Use CI or other indices instead

9.5 Pitfall 5: Confusing Aging with Additives

Case [3]: A weak peak at 1740 cm⁻¹ in a PP part was misidentified as "aging." It was actually the ester group of antioxidant (e.g., Irganox 1010).

Lesson:

• Pre- and post-aging reference spectra are crucial
• Additive peaks are usually stable and do not change over time
• Aging peaks increase with time—comparing time series can distinguish

Summary of This Episode

Thought Questions

1. A PE film after 2 years of use showed CI=0.5 by ATR, but the user reports good performance. Please list possible reasons (at least three) and design verification methods.

2. You need to compare the effects of two light stabilizers on PP aging. Please design a complete experimental plan, including sample preparation, aging conditions, ATR-FTIR testing, CI calculation, and result comparison. Refer to ftir.fun carbonyl page and hydroxyl page to select key peaks.

3. A PA66 part fractures after 3 years of use. The ATR spectrum shows a decrease in the peak at 1540 cm⁻¹ and a new peak at 1710 cm⁻¹. Explain the mechanism and describe how this aging differs from PE photooxidation.

4. For the same PE sample, the CI measured by ATR and the KBr transmission method differs by a factor of 2. What are the possible reasons? Which method is closer to the "true" degree of aging?

5. An outdoor PP part has a CI of 0.8 but has not been stabilized with HALS. Predict its performance trend over the next year and suggest improvement measures.

References

[1] Chinese Plastics Industry Association. Case Studies on Polymer Aging and Failure Analysis. 2023.
https://www.cppia.com.cn/

[2] Gardette J L, Therias S, Mailhot B, Lemaire J. "Photooxidation of Polymers: Mechanisms and Consequences." Polymer Degradation and Stability, 2013, 98(9): 1739–1747. DOI:10.1016/j.polymdegradstab.2013.04.014.

[3] Schnabel W. Polymer Degradation: Principles and Practical Applications. Hanser, 1981. ISBN: 978-3-446-13652-4.

[4] Zweifel H, Maier R D, Schiller M (eds.). Plastics Additives Handbook. 6th ed. Hanser, 2009. Chapter 3 (Antioxidants) and Chapter 4 (Light Stabilizers).

[5] ftir.fun. "Carbonyl (C=O) — FTIR absorption peaks and assignments." FTIR Functional Group Database.
https://ftir.fun/ir/group/car…

[6] Singh B, Sharma N. "Mechanistic implications of plastic degradation." Polymer Degradation and Stability, 2008, 93(3): 561–584. DOI:10.1016/j.polymdegradstab.2007.11.008.

[7] Boubakri A, Elléuch K, Fouvry S, Elleuch N. "FTIR Investigation of Polyurethane Degradation." Polymer Engineering and Science, 2009, 49(9): 1783–1788. DOI:10.1002/pen.21400.

[8] ftir.fun. "Amide — FTIR absorption peaks and assignments." FTIR Functional Group Database.
https://ftir.fun/ir/group/ami…

[9] ftir.fun. "Hydroxyl (O-H) — FTIR absorption peaks and assignments." FTIR Functional Group Database.
https://ftir.fun/ir/group/hyd…

[10] Lacoste J, Carlsson D J. "Gamma-, Photo-, and Thermally-Initiated Oxidation of Polypropylene: AFTIR Study." Polymer Degradation and Stability, 1992, 37(3): 283–296. DOI:10.1016/0141-3910(92)90010-V.

[11] Rivaton A, Gardette J L. "Photooxidation of Polymers." Angewandte Makromolekulare Chemie, 1999, 271: 49–59. DOI:10.1002/(SICI)1522-9505(19990801)271:13.0.CO;2-9.

[12] Kaci M, et al. "Artificial Weathering of Polypropylene: ATR-FTIR Monitoring." Polymer Testing, 2004, 23(8): 893–899. DOI:10.1016/j.polymertesting.2004.04.006.

Next Episode Preview: Ep 25 — Food Industry: Adulteration Detection (Milk, Honey, Olive Oil)
We will shift from industrial manufacturing to food safety, exploring how FTIR uncovers tricks such as milk adulterated with water or urea, honey with high-fructose corn syrup, and olive oil with cheap vegetable oils. Combined with chemometrics (PCA, PLS-DA), FTIR has become the "golden eye" for food adulteration detection.