Determining the degree of plastic decomposition is an important topic in the fields of materials science and environmental engineering. Its methods require a comprehensive analysis combining chemical, physical, and biological properties. Thermogravimetric analysis (TGA) is one of the commonly used methods. By monitoring the mass changes of plastics during programmed temperature increase, the decomposition stage can be accurately determined. For example, polyethylene will go through stages such as melting and main chain breakage at high temperatures. The mass loss rate in different temperature ranges in the TGA curve can intuitively reflect the degree of decomposition – the initial weight loss stage may correspond to the volatilization of additives, while the rapid weight loss range represents the large-scale breakage of polymer chains. In the experiment, if the mass retention rate of a certain type of plastic is 80% at 300°C and drops to 30% at 500°C, it means that it has undergone significant decomposition in the medium and high temperature range, and the residual substance may be carbide or incompletely broken molecular chains.
Fourier transform infrared spectroscopy (FTIR) provides molecular structure-level evidence for the degree of decomposition based on the changes in the characteristic peaks of molecular vibration.
C-C bonds (e.g., C-Cl bonds in polyvinyl chloride) exhibit specific absorption peaks in the infrared spectrum. During decomposition, the intensity of these peaks decreases or shifts, reflecting the breakdown of chemical bonds. For example, during the degradation of polylactic acid ( PLA ), the absorption peak of the ester bond (around 1750 cm⁻¹ ) gradually decreases over time, while the peak intensity of the hydroxyl group ( around 3400 cm⁻¹ ) increases, indicating that the ester bond breaks down, generating small hydroxyl-containing products. By comparing spectra at different degradation stages, the proportion of residual polymer can be quantitatively analyzed, providing data support for assessing material stability.
Gel permeation chromatography (GPC) determines the degree of degradation by measuring changes in molecular weight distribution. Plastic degradation is essentially a process of molecular weight reduction caused by the breakage of polymer chains. GPC can separate components of different molecular weights and calculate the weight-average molecular weight (Mw) and number-average molecular weight (Mn). If the Mw of a batch of polypropylene (PP) is 500,000 before aging testing and drops to 200,000 after the test, with a broader molecular weight distribution, this indicates significant degradation, with molecular chain breakage producing more low-molecular-weight fragments. This method is particularly suitable for assessing the performance degradation of plastics after long-term use or environmental exposure, providing a key basis for predicting material lifespans.
Scanning electron microscopy ( SEM) reveals the microscopic characteristics of plastic decomposition. While the surface of undecomposed plastic is typically smooth and uniform, decomposed materials may exhibit cracks, holes, or flaking fragments. For example, under hydrolysis conditions, polyethylene terephthalate (PET) gradually forms erosion pits on its surface. As degradation progresses, the pits expand and interconnect, ultimately leading to the collapse of the material structure. By observing these microscopic changes with SEM and combining them with image analysis software to calculate surface roughness or porosity, the degree of decomposition can be indirectly quantified. Furthermore, combining SEM with energy dispersive spectroscopy (EDS) can detect the introduction of foreign elements (such as nitrogen and phosphorus from microbial metabolites) during the decomposition process, providing clues to the decomposition mechanism.
Chemical titration, a classic chemical method for determining the extent of decomposition, quantitatively analyzes the decomposition products of specific plastics. For example, the decomposition of polyvinyl chloride (PVC) releases hydrogen chloride (HCl). By titrating the decomposition gases with a standard sodium hydroxide solution, the amount of HCl released can be calculated based on the amount of base consumed, and the proportion of PVC decomposition can be inferred. For polyester plastics, hydrolytic degradation produces carboxyl groups. Phenolphthalein can be used as an indicator, and the degree of degradation can be determined by titrating the carboxyl group content. This method is simple to perform and low-cost, making it suitable for rapid testing in laboratories or industrial production. However, it should be noted that titration results may be affected by interfering substances and require cross-validation with other methods to ensure accurate determination.
