The carbonyl index (CI) is the most widely used quantitative indicator of photo-oxidative degradation in photodegradable plastics such as polyethylene and polypropylene. Determined by Fourier-transform infrared (FTIR) spectroscopy, it measures the growth of carbonyl-containing oxidation products after ultraviolet or natural weathering exposure. This article explains the testing principle, sample requirements, measurement procedures, calculation methods, applicable ISO, ASTM and GB standards, and typical applications of carbonyl index determination for photodegradable materials.
Technical Overview
Photodegradable materials are polymers, predominantly polyethylene (PE), polypropylene (PP) and their copolymers, formulated with pro-oxidants or photosensitizers so that they undergo controlled degradation under the combined action of sunlight, oxygen, heat and moisture. Ultraviolet photons excite chromophoric impurities and carbonyl defects in the polymer backbone, initiating free-radical chain oxidation. Propagation and branching reactions form hydroperoxides that decompose into alkoxy and peroxy radicals, and subsequent chain scission through Norrish Type I and Type II photochemical reactions generates ketones, aldehydes, carboxylic acids, esters and lactones along and at the ends of the polymer chains.
All of these oxidation products contain the carbonyl (C=O) functional group, whose characteristic stretching vibration absorbs infrared radiation in the region of approximately 1710 to 1740 cm⁻¹. As degradation proceeds, the intensity of this absorption band increases in near proportion to the extent of oxidation. The carbonyl index is therefore defined as the ratio of the infrared absorbance, measured either as peak height or integrated band area, of the carbonyl band centered near 1715 cm⁻¹ to the absorbance of an internal reference band that remains essentially unchanged during degradation, such as the methylene scissoring vibration at 1465 cm⁻¹. A common expression is:
CI = (A₁₇₁₅ / A₁₄₆₅) × 100
Because the reference band corrects for variations in specimen thickness and optical scattering, the carbonyl index provides a reproducible, dimensionless measure of the degree of photo-oxidation. It is the principal analytical metric in accelerated and outdoor weathering studies of photodegradable plastics and constitutes the first tier of recognized multi-tier degradation testing schemes such as ASTM D6954, in which abiotic oxidation characterized by the carbonyl index is followed by molecular weight and biodegradation assessments.
Testing Samples
Carbonyl index determination can be performed on virtually any polyolefin-based specimen configuration, provided the infrared sampling mode is matched to the sample geometry. Common sample types submitted to accredited third-party laboratories include:
Polymer Films
Thin blown or cast films of LDPE, LLDPE, HDPE or PP, typically 20 to 100 µm thick, are the preferred geometry because they can be measured directly in transmission mode, where the Beer-Lambert law applies and the full specimen thickness is interrogated. Agricultural mulch films and degradable packaging films are the most frequent examples.
Molded Plaques and Sheets
Injection-molded or compression-molded plaques and thicker sheets are generally analyzed by attenuated total reflectance (ATR) FTIR on the exposed surface, or by transmission after microtoming a thin section. Since photo-oxidation is concentrated in the outer few micrometres of an exposed article, the surface selectivity of ATR is chemically appropriate for these geometries.
Granules, Powders and Reground Material
Pellets, powders, millings and regrind recovered from aged articles can be measured as potassium bromide pellets or by pressing the material directly against the ATR crystal under a defined clamping force, provided the irradiated fraction is representative of the bulk material.
Finished Articles and Field-Exposed Specimens
Commercial products, mulch films recovered from the field after defined cultivation periods, and litter or failure-analysis samples are tested non-destructively by ATR at multiple locations on both faces to capture the heterogeneous spatial distribution of oxidation.
Testing Items
A standard test program for carbonyl index determination typically reports the following items:
- Initial (unaged) carbonyl index as the baseline value of the material as received.
- Carbonyl index after defined exposure intervals, for example 50, 100, 200 and 400 hours of accelerated weathering or defined periods of outdoor exposure.
- Incremental change in carbonyl index (ΔCI) and carbonyl growth rate between successive exposure points.
- Carbonyl band position and band-shape analysis, distinguishing ketone (~1715 cm⁻¹), carboxylic acid (~1710 cm⁻¹), aldehyde (~1730 cm⁻¹) and ester (~1740 cm⁻¹) contributions.
- Complementary oxidation indices where required, such as the hydroxyl index (broad band at 3200 to 3600 cm⁻¹), vinyl index (910, 990 and 888 cm⁻¹ Norrish Type II markers) and the crystalline-to-amorphous band ratio (730/720 cm⁻¹).
- Associated degradation indicators commonly requested in combination with the carbonyl index, including retention of tensile elongation at break per ASTM D3826, molecular weight distribution by gel permeation chromatography, and degree of embrittlement.
Testing Methods
Photo-Oxidative Exposure
Specimens are degraded under standardized light sources before infrared measurement. Accelerated laboratory exposure is performed with xenon arc lamps or fluorescent ultraviolet lamps in accordance with ISO 4892-2, ISO 4892-3, ASTM G154, ASTM G155 and ASTM D5208, with the GB/T 16422 series providing the corresponding Chinese national adoption. Natural weathering is conducted per ISO 877, ASTM G7 and ASTM D5272. Typical fluorescent UV cycles combine UV irradiation at 340 nm (0.89 W/m²) at 60 °C with condensation at 50 °C, while typical xenon cycles apply 0.35 W/m² at 340 nm with a black panel temperature near 63 °C and defined water-spray intervals. Thermal oxidation aging of oxo-degradable materials may additionally be performed per ASTM D5510.
FTIR Spectroscopic Measurement
Spectra are collected in transmission mode for thin films with a maximum absorbance below approximately 1.0 to preserve Beer-Lambert linearity, and in ATR mode for thicker, opaque or irregular specimens using a diamond or zinc selenide crystal with a sampling depth of roughly 1 to 2 µm. Typical acquisition parameters cover the mid-infrared range of 4000 to 400 cm⁻¹ at 4 cm⁻¹ resolution with at least 32 co-added scans against a fresh background. Measurements are taken at three to five positions on at least three replicate specimens, on both the exposed and unexposed faces, and specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity per ISO 291 before analysis.
Calculation and Data Processing
A linear baseline is constructed across the carbonyl region, commonly between local minima near 1500 cm⁻¹ and 1820 to 1850 cm⁻¹. The carbonyl signal is taken as the peak height at 1715 ± 5 cm⁻¹ or as the integrated band area between defined limits, and the reference signal is taken analogously at 1465 cm⁻¹. The carbonyl index is calculated as the normalized ratio, typically multiplied by 100. The test report must state the calculation basis, the exact band positions and integration limits, the baseline construction method, the sampling mode and the complete exposure conditions, since absolute CI values are only comparable within a consistently defined protocol.
Quality Control and Validation
- Instrument performance verification using a certified polystyrene reference film for wavenumber accuracy and resolution.
- Linearity verification in accordance with the Beer-Lambert law by selecting appropriate film thickness or employing ATR sampling.
- Replicate measurement of at least three positions per specimen with results reported as mean and standard deviation.
- Consistent use of the same reference band and calculation formula throughout a comparative study.
- Full documentation of exposure dose, spectral parameters and specimen handling to ensure traceability.
Applications
- Agricultural mulch films: predicting the timing of embrittlement, scheduling film recovery or soil incorporation, and comparing the degradation schedules of competing formulations.
- Degradable packaging and single-use articles: substantiating photodegradation performance claims and completing Tier 1 abiotic oxidation characterization within the ASTM D6954 testing scheme.
- Additive and formulation development: ranking the efficacy of UV stabilizers and antioxidants, and optimizing pro-oxidant loading levels.
- Polymer research: quantitative kinetics modeling of photo-oxidation and service-life prediction for polyolefin products.
- Failure analysis: identifying photo-oxidative degradation as the root cause of in-service embrittlement, cracking or discoloration.
- Regulatory and conformity support under GB/T 20197 and comparable degradable-plastic material specifications.
FAQ
What exactly is the carbonyl index?
The carbonyl index is a dimensionless FTIR parameter equal to the ratio of the infrared absorbance of the carbonyl band near 1715 cm⁻¹ to that of an internal thickness-reference band, usually at 1465 cm⁻¹, often multiplied by 100. It quantifies the concentration of carbonyl-bearing oxidation products and therefore tracks the progress of photo-oxidative degradation.
Why is the band near 1715 cm⁻¹ selected?
This wavenumber corresponds to the stretching vibration of ketone carbonyl groups, which are the dominant stable oxidation products in photo-degraded polyolefins, with overlapping contributions from carboxylic acids, aldehydes and esters in the surrounding 1710 to 1740 cm⁻¹ region. The band is absent or minimal in unaged polyethylene and polypropylene, making its growth a sensitive and specific degradation marker.
Which reference band should be used?
The methylene scissoring band at 1465 cm⁻¹ is the most common choice because it is intense, well resolved and largely unaffected by oxidation. Alternatives include the C-H stretching bands near 2916 and 2848 cm⁻¹ and the crystalline rocking doublet near 730 and 720 cm⁻¹. Whichever band is chosen must be used consistently, since absolute CI values are not interchangeable between different reference definitions.
Can thick, opaque or finished products be tested?
Yes. ATR-FTIR probes only the outer 1 to 2 µm of the specimen, which is precisely where photo-oxidation is concentrated, so molded parts and finished articles can be measured directly. For through-thickness analysis, thin microtomed sections may be prepared for transmission measurement.
How does the carbonyl index relate to embrittlement and biodegradation?
A rising carbonyl index reflects chain scission and correlates with declining molecular weight and loss of elongation at break, ultimately producing embrittlement and fragmentation. However, the carbonyl index alone does not demonstrate biodegradation; for oxo-degradable systems it defines the abiotic oxidation stage, after which subsequent biodegradation must be verified by separate respirometric testing.
Which standards govern exposure and measurement?
Laboratory exposure is governed by ISO 4892-2 and ISO 4892-3, ASTM G154, ASTM G155 and ASTM D5208, natural weathering by ISO 877, ASTM G7 and ASTM D5272, thermal aging by ASTM D5510, end-point tensile evaluation by ASTM D3826, the overall tiered degradation scheme by ASTM D6954, and material classification by GB/T 20197, with the GB/T 16422 series mirroring ISO 4892 for laboratory light sources.