Intrinsic Viscosity (IV) Determination Of Recycled Pet By Nir Spectroscopy

Introduction

Intrinsic viscosity measurements are a standard quality control procedure for industries dealing with virgin or recycled poly(ethylene terephthalate) (rPET). This is because intrinsic viscosity is linked to the average molar mass, which affects both processing and the final mechanical properties of the polymer. The relationship between intrinsic viscosity and other properties is evident when the polymer chain is linear and the molar mass has low dispersity, as is typical with PET. Traditional methods for determining PET's intrinsic viscosity involve the use of chemical techniques, which require toxic and costly solvents, are time-consuming, prone to operational errors, and generate substantial hazardous waste.

The crucial of the NIR method in rPET industries

In the industry, NIR spectroscopy is vital for controlling the viscosity of rPET, ensuring high-quality production and efficient processing. NIR spectra offer rapid, non-invasive analysis of rPET’s molecular structure, closely linked to its viscosity. This real-time monitoring allows for immediate adjustments, preventing issues like material degradation or contamination that could compromise product integrity. By maintaining optimal viscosity, NIR helps ensure that rPET meets stringent industry standards for performance, particularly in applications like packaging, especially in food-grade applications. Additionally, it supports sustainability efforts by reducing waste and enhancing the overall efficiency of the recycling process.

Method and Application

The method is based on collecting absorbed spectroscopies into a database and using machine learning to extract precious results. The absorbance spectra are converted from reflectance by log(1/R).

The FTIR spectrometer operated In the near-infrared region (900 nm – 1700 nm), Figure 1 illustrates the NIR signatures for PET samples. Regardless of the life cycle stage, PET samples exhibit distinct characteristic peaks. Both primary and secondary raw material samples display identifiable absorption peaks at 1130 nm, 1170 nm, 1420 nm, and 1660 nm. Spectral signatures of waste samples in flakes reveal prominent peaks at 1130 nm and 1660 nm, while the peaks at 1170 nm and 1420 nm are less pronounced. Figure 1b demonstrates that the signals were processed using a specialized technique to enhance the sharpness of the peaks, facilitating easier calibration steps.

Figure 1. Representative NIR signatures of PET samples on conveyor belt. a) Raw spectra, b) processed spectra.

The intrinsic viscosities were determined by the standard chemical method (ASTM D 4603-01) and a very good correlation for values in the range from 0.346 to 0.780 dL g−1 (relative viscosity values ca. 1.185–1.449) was observed. After collecting the database, it will be used for the calibrating step to determine the viscosity of samples. Meaning the measurement of a viscosity standard were measured at the same of the reflectance spectrum. The calibration and predictive (validation) values were practically coincident (Figure 2) indicating the good predictive capacity of these models, with R² being 0.985.

Figure 2. The accuracy of the training results of PET and PVC.

Equipment

In this application, three models of NIRQuest spectrometers are utilized: the NIRQuest+1.7 (covering 900 to 1650 nm), the NIRQuest+2.2 (covering 900 to 2120 nm), and the NIRQuest+2.5 (covering 900 to 2450 nm). Among these, Ocean Optics recommends the NIRQuest+2.2 model as the most suitable for chemical concentration measurements due to its optimal spectral range, which provides a balance between sensitivity and wavelength coverage for detecting key absorption features. The NIRQuest+2.2 offers a minimum integration time of just 1 ms, enabling fast data acquisition, which is essential for high-throughput or real-time applications. Additionally, it features an SMA input for fiber optic connections, ensuring compatibility with a wide range of sampling accessories and configurations. This combination of speed, precision, and flexibility makes the NIRQuest+2.2 an ideal choice for demanding analytical tasks in various chemical analysis applications.