Photodegradation Carbonyl Index Determination: FTIR-Based Assessment of Polymer UV Degradation

The carbonyl index (CI) is a quantitative indicator of polymer photodegradation, measuring the accumulation of carbonyl-containing oxidation products that form when plastics are exposed to UV radiation, heat, and moisture. Determined primarily by Fourier-transform infrared (FTIR) spectroscopy, this test is essential for evaluating the oxidative degradation behavior of polyethylene, polypropylene, and oxo-degradable materials. Accredited third-party laboratories perform carbonyl index determination in combination with laboratory weathering exposures conducted according to ISO 4892 and ASTM D6954 to support product development, quality control, and environmental performance claims.

Technical Overview

Photodegradation of polyolefins such as polyethylene (PE) and polypropylene (PP) proceeds through a free-radical chain oxidation mechanism initiated by ultraviolet radiation. As UV photons break polymer chains and activate oxygen, secondary reactions generate ketones, aldehydes, carboxylic acids, esters, and lactones within the polymer matrix. These oxidation products all contain the carbonyl functional group (C=O), which exhibits a characteristic infrared absorption band between approximately 1710 and 1725 cm⁻¹.

The carbonyl index normalizes the absorbance of this carbonyl band against a stable internal reference band, typically the C-H bending vibration of the polymer backbone near 1465 cm⁻¹ (or 1377/1368 cm⁻¹ for CH₃ groups in polypropylene). This normalization corrects for variations in specimen thickness and measurement geometry, allowing reliable comparison of the degree of oxidation between unexposed and weathered materials.

A rising carbonyl index directly reflects progressive photo-oxidative degradation. As the index increases, polymers typically exhibit chain scission, reduced molecular weight, surface cracking, embrittlement, and fragmentation. For oxo-degradable plastics, the carbonyl index serves as the primary acceptance criterion in the tiered evaluation scheme of ASTM D6954, in which initial abiotic oxidation is verified before subsequent biodegradation and ecotoxicity testing is performed.

Testing Samples

Polyethylene (PE) Films and Bags

Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) materials, including carrier bags, agricultural mulch films, greenhouse films, and packaging films, represent the most common sample categories. Films are typically tested at thicknesses between 10 and 100 micrometers for optimal transmission FTIR measurement.

Polypropylene (PP) Products

Woven tapes, nonwoven fabrics, injection-molded articles, and cast or biaxially oriented films made from PP homopolymer or copolymer grades can be evaluated. The reference band selection for PP differs slightly due to its methyl-group branching.

Oxo-Degradable and Pro-Oxidant Modified Compounds

Materials incorporating pro-oxidant additives such as transition metal stearates are specifically designed to accelerate carbonyl formation under UV exposure. These compounds require carbonyl index monitoring to verify that degradation proceeds within the intended timeframe.

Pellets, Plaques, and Molded Specimens

For formulation screening and research purposes, resin pellets can be compression-molded into standardized plaques prior to exposure. Molded specimens, extruded sheets, and finished parts of nearly any geometry can be tested using attenuated total reflectance (ATR) accessories when thickness or opacity prevents transmission analysis.

Testing Items

The following parameters are typically included in a comprehensive photodegradation carbonyl index evaluation:

• Initial carbonyl index of unexposed material
• Carbonyl index after defined UV exposure intervals
• Carbonyl index growth rate and change (ΔCI) versus exposure dose
• Peak absorbance values at specified wavenumbers (e.g., 1715 cm⁻¹ carbonyl band, 1465 cm⁻¹ reference band)
• Hydroxyl index (hydroperoxide and hydroxyl absorption near 3400 cm⁻¹)
• Vinyl index (unsaturation absorption near 1640 cm⁻¹ or 910 cm⁻¹)
• Complementary degradation indicators: tensile strength and elongation at break retention, molecular weight distribution by gel permeation chromatography (GPC), surface cracking assessment, mass loss, and fragmentation behavior

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut from representative sample areas, cleaned of surface contaminants, and conditioned in a standard atmosphere in accordance with ISO 291 or ASTM D618 (typically 23 °C and 50% relative humidity) before baseline FTIR measurement. Replicate specimens are prepared for each exposure interval to ensure statistical validity.

Laboratory Weathering Exposure

Controlled photo-oxidation is induced using artificial weathering devices prior to carbonyl measurement. Common exposure protocols include:

• Xenon arc lamp exposure with filtered radiation and water spray cycles per ISO 4892-2, ASTM G155, or ASTM D2565
• Fluorescent UV lamp exposure with condensation cycles per ISO 4892-3, ASTM G154, or ASTM D5208
• Combined oxidation and thermal aging sequences per ASTM D5510 for oxidatively degradable plastics
• Direct outdoor weathering under natural sunlight per ISO 877 or ASTM G7 where real-service correlation is required

Specimens are removed at sequential exposure intervals to generate a carbonyl growth curve as a function of radiant exposure (kJ/m² at 340 nm) or elapsed time.

FTIR Spectroscopic Measurement

Infrared spectra are collected over the range of 4000 to 400 cm⁻¹ at a resolution of 4 cm⁻¹ with a minimum of 32 scans per spectrum. Thin transparent films are measured in transmission mode, while thicker, opaque, or filled materials are measured using an ATR crystal. Baseline correction is applied consistently across all spectra, and both surfaces of film specimens may be analyzed since oxidation concentrates near the irradiated face.

Calculation and Reporting

The carbonyl index is calculated from the ratio of the maximum absorbance in the carbonyl region to the absorbance of the chosen reference band:

CI = (A₁₇₁₅ / A₁₄₆₅) × 100, where A₁₇₁₅ is the absorbance at the carbonyl maximum (approximately 1710–1720 cm⁻¹) and A₁₄₆₅ is the absorbance of the CH₂ scissoring reference band. For polypropylene, reference bands at 1377 cm⁻¹ or 1165 cm⁻¹ are frequently substituted. Test reports present the carbonyl index at each exposure point, the calculated rate of increase, replicate averages, and full spectral documentation.

Applications

Photodegradation carbonyl index determination serves a broad range of industrial and research purposes:

• Verification of degradation performance claims for oxo-degradable packaging and carrier bags under the tiered framework of ASTM D6954
• Design and lifetime prediction of agricultural mulch films that must retain strength during the growing season and fragment afterward
• Screening of UV stabilizers, hindered amine light stabilizers (HALS), and antioxidant packages by comparing carbonyl growth kinetics
• Durability qualification of outdoor polyolefin products including greenhouse films, geotextiles, ropes, nets, pipes, and construction sheeting
• Regulatory and compliance support, including assessment of materials subject to restrictions on oxo-degradable plastics in certain jurisdictions
• Academic and industrial research into polymer photo-oxidation mechanisms, recycling stability, and microplastic fragmentation behavior
• Quality control of incoming resins, masterbatches, and finished goods for consistent degradation or stabilization performance

FAQ

What is the carbonyl index and why is it important?

The carbonyl index is a dimensionless ratio comparing the infrared absorbance of carbonyl oxidation products to a stable reference band of the polymer. It is the most widely used quantitative metric of polyolefin photo-oxidation, because carbonyl groups accumulate in direct proportion to the extent of UV-induced degradation.

What carbonyl index value indicates significant degradation?

There is no universal pass/fail threshold, since the critical value depends on the polymer, application, and specification. In practice, materials are considered to be in an advanced state of oxidation when the carbonyl index has increased severalfold from its initial value and approaches a plateau, which typically coincides with embrittlement and the onset of fragmentation. Some procurement specifications require the carbonyl index to exceed a defined minimum after a specified exposure dose.

Does a high carbonyl index prove that a plastic is biodegradable?

No. The carbonyl index confirms abiotic oxidative degradation only. Under ASTM D6954, oxidation testing constitutes Tier 1; biodegradation must be independently demonstrated in Tier 2 through tests such as ISO 17556 or ASTM D5988, followed by ecotoxicity assessment in Tier 3.

Should transmission or ATR FTIR be used?

Transmission mode is preferred for thin, transparent films because it probes the full specimen thickness. ATR is suitable for thick, opaque, or filled materials and measures a surface layer of only a few micrometers, which is advantageous when studying surface oxidation gradients. The method selected must remain consistent throughout a given study.

Which reference band is correct for the calculation?

The 1465 cm⁻¹ CH₂ scissoring band is the conventional reference for polyethylene, while 1377 cm⁻¹ or 1165 cm⁻¹ bands are common for polypropylene. Because the choice of reference band changes the absolute index value, the reference used must always be stated in the test report.

What sample quantities and forms are required?

Typically, specimens of at least 20 × 20 mm are needed, with sufficient replicates for each planned exposure interval. Films, cut pieces, molded plaques, and sections taken from finished articles are all acceptable. Accredited third-party laboratories can advise on specific dimensional requirements based on the selected FTIR mode and weathering protocol.

How long does the test take?

Turnaround depends on the exposure protocol. Carbonyl index measurement itself is rapid, but laboratory weathering may run from several days to several weeks or months depending on the required radiant exposure dose and the degradation rate of the material.