Carbonyl Index

Infrared Analysis

Fourier transform infrared spectrometers measure the absorbance of carbonyl stretching vibrations at approximately one thousand seven hundred and fifteen wavenumbers. Technicians compare this absorbance value against a reference peak that remains unchanged during degradation, such as the methylene stretching band at one thousand four hundred and sixty wavenumbers. The ratio between these two absorption bands yields the index value.

Since the reference peak adjusts for variations in specimen thickness, the resulting value can be compared across different samples.

Degradation Tracking

Outdoor exposure to ultraviolet light initiates chain reaction oxidation, which generates carbonyl, hydroxyl, and ester groups. As the index increases, the molecular weight of the polymer decreases due to progressive chain scission. This measurement is crucial for evaluating weathering performance in agricultural films, automotive trim, and outdoor furniture.

Mechanical Loss

High index values correspond to severe embrittlement of the plastic matrix. Molded parts lose tensile strength and impact resistance when the index exceeds a critical threshold. A microcracked surface layer develops, which accelerates mechanical failure under load.