Meaning
Infrared absorption bands denote specific chemical bonds vibrating under infrared irradiation, producing unique spectral signatures for chemical identification. On an infrared spectrum of condensation polymers, an ester linkage peak represents the carbonyl stretch vibration located between 1715 and 1750 reciprocal centimetres. Fourier-transform infrared spectroscopy isolates this band to confirm the presence of ester groups in polyesters like polyethylene terephthalate, polybutylene terephthalate, and polycarbonates.
Spectral analysis ceases to provide meaningful molecular mass information once the polymer chains degrade into shorter oligomers that still retain intact ester groups.
Hydrolysis Tracking
Moisture contamination during high-temperature melt processing causes hydrolytic chain scission directly at vulnerable carbonyl sites. When condensation resins enter an injection barrel without thorough desiccant drying, water molecules sever the ester bonds, yielding carboxyl and hydroxyl end groups. Spectroscopic monitoring tracks the relative decrease of the ester linkage peak alongside an elevation in the broad hydroxyl region.
Moulded parts suffering from this breakdown exhibit severe brittleness, low tensile impact strength, and premature environmental stress cracking during service.
Contamination Detection
Polyolefin recyclers monitor infrared absorption to detect cross-contamination from polyester or polyurethane packaging films. Clean polyethylene streams show zero baseline response at ester frequencies, meaning any detectable ester linkage peak indicates incomplete sort separation. Compounding extruders running contaminated feedstock experience phase separation, poor interfacial adhesion, and surface delamination during injection moulding.
Infrared absorbance thresholds on raw flake deliveries prevent substandard flake blends from entering manufacturing operations.
Spectrometric Calibration
Quantitative determination of polymer degradation requires baseline correction against an internal reference band that remains unaffected by processing conditions. Aromatic ring vibrations provide an invariant standard against which the ester absorbance is calculated as a ratio. Spectral resolution settings must avoid peak truncation, ensuring that absorbance ratios accurately identify resin changes between incoming resin lots.
