Application Note. Redefining PET Quality Control: Automated Intrinsic Viscosity Analysis in Production Environments

By Polymer Char, Valencia, Spain. 
F. Cordero, A. Cárdenas, N. Fernández, A. Ortín

Redefining PET Quality Control: Automated Intrinsic Viscosity Analysis in Production Environments

1. Introduction

The production and transformation of polyethylene terephthalate (PET), especially in high-throughput environments such as packaging or recycling plants, demands precise and repeatable control of material properties throughout the process. One of the most critical parameters for assessing PET quality is the determination of its viscosity in a dilute solution using capillary viscometers. Hence, the intrinsic viscosity (IV), which directly reflects the polymer’s molecular weight and its suitability for applications like bottle-grade resin or textile fibers is one of the most important parameters to control the quality of this material¹.

In most industrial settings, the viscosity of PET in dilute solution is determined according to the ISO 1628-5 standard using traditional Ubbelohde-type glass capillaries where intrinsic, reduced and inherent viscosity could be determined. This method involves dissolving the polymer in a solvent mixture at high temperature, followed by viscosity measurement in glass capillary viscometers². While reliable, this approach is highly manual, time-consuming, and sensitive to operator technique. Moreover, as batch frequency and diversity increase, so do the risks of inconsistencies in sample preparation, dissolution timing, and endpoint determination.

During PET production, especially in the Solid-State Polycondensation phase (SSP), small variations in processing time, temperature, or moisture control can significantly affect the polymer’s degree of polymerization and consequently, its intrinsic viscosity³. Since IV directly reflects molecular weight, even a few hours’ difference in processing can result in measurable changes in IV values, indicating shorter or longer polymer chains. Critical parameters such as residual moisture, reaction kinetics, and catalyst activity all influence this outcome⁴. Yet, the traditional Ubbelohde method, while accurate, is not agile enough to monitor these dynamic shifts in real time or to support rapid quality decisions across shifting process conditions.

Figure 1.- SSP schematic process

An appealing workaround is to maintain reliance on the traditional glass capillary method but attempt to streamline operations by simplifying steps or batching samples. However, this partial solution still faces serious drawbacks: manual dissolution still introduces time delays and variability; results are not available fast enough to impact dynamic production stages; and the solvent handling process carries safety and environmental burdens.

As an alternative, this study explores the application of the automated analyzer for the determination of the viscosity of PET in dilute solution using a dual capillary relative viscometer, offering a faster, safer, and more reproducible way to measure its viscosity even directly from production samples with no pre-treatment. By eliminating manual steps, automating dissolution and measurement, and integrating a dual-capillary system, this approach enhances consistency and enables rapid quality assessment across varying PET grades, including recycled content. Owing to these advantages, the methodology has been formally recognized and incorporated into ISO 1628-1, and is currently under consideration for adoption within ISO 1628-5, which specifically addresses polyesters such as PET.

2. Experimental

2.1 Overview
This study evaluated the viscosity of PET in dilute solution of five samples using a fully automated solvent-based system focusing on Intrinsic Viscosity (IV). The goal was to assess whether an alternative automatic analytical workflow could provide reliable, reproducible IV measurements directly from production samples, with no pre-conditioning steps such as drying or grinding.
The experiment was designed to compare samples of recycled PET (rPET) and virgin PET (vPET), while also capturing the effect of polymerization progression at different stages of material synthesis with samples collected from the polymerization process (iPET). The efflux time method using a Ubbelohde glass capillary according to ISO 1628-5 was used as a reference for method validation, as this remains the dominant criterium in the industry.

2.2 Sample Information
The samples were sourced from an industrial PET producer. Each sample was collected at different time points during real manufacturing processes, representing a variety of polymerization levels and processing histories. The samples were:

• rPET-7AM – collected from the process at 7:00 AM
• rPET-11PM – collected from the process at ~16 hours later.
•  iPET-A and iPET-B – industrial polymerization process intermediates
•  vPET– commercial virgin reference PET for general resin propose.

These samples were analyzed exactly as received, with no modifications or physical preparation steps applied, simulating a real-world industrial QA/QC context.

2.3 Sample Preparation
Each sample was prepared according to the ISO 1628-5 guidance for dilute solution viscosity measurements. A 5 mg/mL solution was prepared using a 60:40 v/v mixture of phenol (Scharlab, Pharmpur grade) and 1,1,2,2-tetrachloroethane (TCE 98.5% Fischer Scientific). The polymer was placed into 30 mL vials and subjected to the following controlled dissolution conditions:

• Dissolution temperature: 120 °C
• Magnetic stirrer speed: 400 rpm
• Dissolution time: 30 minutes

This procedure ensured complete dissolution of the polymer chains without the risk of thermal degradation. The protocol was designed to minimize variability and operator influence, ensuring high consistency across samples.

2.4 Instrument and Analytical Procedure
The analysis was carried out using the IVA Versa instrument (Polymer Char), which operates based on differential pressure using a dual capillary relative viscometer. This setup enables real-time measurement of relative viscosity by comparing the flow resistance of the solvent and sample solution simultaneously.
From this, the system automatically reports:

• Viscosity number (VN) also called reduced viscosity,
• Intrinsic viscosity (IV) via the Solomon-Ciutà equation, and
• Additional viscosity parameters such as inherent viscosity.

Furthermore, the software allows the application of alternative mathematical models beyond Solomon-Ciutà, ensuring compatibility with different analytical protocols and regulatory standards.

Analytical parameters:

• Injection volume: 200 µL
• Flow rate: 1.0 mL/min
• Capillary temperature: 25 °C

Each sample was analyzed in triplicate, with two injections per replicate, allowing for statistical evaluation of method precision and repeatability.

3. Results and Discussion

3.1 Comparative Analysis of Automated and Manual Viscosity Determination
A comparative evaluation was performed between an automated viscometer (IVA Versa) and the efflux time method using a Ubbelohde glass capillary according to ISO 1628-5. Five PET samples, representing recycled grades, polymerization process and virgin grade were analyzed directly as received from production, without pre-conditioning. The purpose of this comparison was to assess consistency, process sensitivity, and potential systematic differences between the two methodologies.

3.2 Comparative Results Interpretation

Magnitude and direction of differences across all samples: The automated method produced IV values marginally higher than those obtained by the manual procedure. The observed differences ranged from +0.007 to +0.037 dL/g, with an average absolute difference of +0.021 dL/g. This consistent positive offset may be attributed to the automated system’s higher sensitivity to high-molecular-weight fractions and its ability to minimize IV loss from manual handling or solution degradation.

Detection of Process-Driven Changes: The recycled PET samples rPET-7AM and rPET-11PM were collected at different stages of a solid-state polycondensation (SSP) process, with a time interval of approximately 16 hours. In recycling operations, SSP is widely employed not only to increase molecular weight for virgin PET production but also to restore intrinsic viscosity in rPET after thermal and mechanical degradation during prior processing⁵. The automated system detected a clear increase of +0.191 dL/g, consistent with the expected chain extension and molecular weight recovery achieved during SSP and measured this change with repeatability levels of RSD ≤ 1.27%.

According to the industrial grade variability samples (iPET), the largest deviation between methods was observed for sample iPET-A (+0.0375 dL/g, ~6.5% relative difference). This may reflect differences in sensitivity to stabilizers, low-molecular-weight fractions, or dissolution efficiency. The sample iPET-B had a much lower difference than iPET-A where +0.0106 dL/g (~1.4%) was observed. These results suggest that the composition and heterogeneity of certain batches, such as the presence of stabilizers or low-molecular-weight fractions in iPET-A, can influence the measurement sensitivity⁴, whereas iPET-B appears more consistent between automated and manual methods. Overall, automated differential viscometry may more accurately capture the effective polymer chain length in specific industrial formulations.

A striking observation is that vPET, as the only virgin PET sample, not only exhibited the lowest relative standard deviation (RSD: 0.28%) among all batches analyzed but also demonstrated the automated method’s ability to detect subtle differences in polymer chain length with high reproducibility⁶. This underscores that automation not only enhances efficiency but also provides confidence in applications where material uniformity is critical, such as supplier certification and bottle-grade resin quality assurance. The stability and precision observed in vPET stand in clear contrast to the greater variability seen in industrial recycled PET samples, highlighting how additives or prior processing in recycled polymers can influence viscometric measurements.

Figure 2.- Intrinsic Viscosity plot comparation between the PET samples

3.3 Relevance to Industrial Quality Assurance
Both methodologies can identify grade-to-grade differences and track process-related changes in IV. However, the automated viscometer solvent-based approach offers distinct advantages for industrial quality control:

• Improved reproducibility by minimizing operator handling and standardizing dissolution and measurement steps.
• Greater sensitivity to subtle shifts in molecular weight, supporting more accurate process adjustments.
• Direct compatibility with production samples, reducing total analysis time and enabling more responsive decision-making.

These attributes are particularly relevant in environments handling recycled PET (rPET) or multiple product grades, where variability is higher and process control requires rapid, precise feedback.

4. Conclusions

The automated solvent-based viscometer has achieved high repeatability (RSD ≤ 1.27%), well within the commonly accepted precision threshold for intrinsic viscosity testing in polymer quality control laboratories, where an RSD ≤ 2% is considered excellent. This level of consistency enables the reliable detection of process-driven IV variations as small as 0.007 dL/g in production-representative PET samples⁷.

Across all PET grades tested (recycled, industrial, and virgin) the automated system consistently reported slightly higher IV values (+0.021 dL/g on average), which may reflect improved sensitivity to high-molecular-weight fractions and reduced losses from manual handling or solution degradation.

The capability to directly analyze production samples without pre-conditioning, combined with faster throughput and lower solvent consumption, positions the automated method as a robust and sustainable alternative for routine PET quality assurance, particularly in processes involving solid-state polycondensation (SSP) in both virgin and recycled PET.

5. References

1. A. Elamri et al. (2015)., Characterization of Recycled / Virgin PET Polymers and their Composites. American Journal of Nano Research and Application; 3(4-1): 11-16. doi: 10.11648/j.nano.s.2015030401.13
2. M. Asensio et al. (2020)., Rheological modification of recycled poly(ethylene terephthalate): Blending and reactive extrusion. Polymer Degradation and Stability Volume 179, doi.org/10.1016/j.polymdegradstab.2020.109258
3. Y. Ma et al. (2003)., Solid-state polymerization of PET: influence of nitrogen sweep and high vacuum. Polymer 44 (2003) 4085–4096. doi:10.1016/S0032-3861(03)00408-7
4. Ganapathi et al (2024)., Achieving bottle‐grade poly ethylene terephthalate ‐like properties from blends. Polymer International. DOI 10.1002/pi.6633
5. T. Y. Kim et al (2003)., Solid-State Polymerization of Poly(ethylene terephthalate). I. Experimental Study of the Reaction Kinetics and Properties. Journal of Applied Polymer Science, Vol. 89, 197–212
6. A. Cárdenas (2024)., Streamlining PET Viscosity Measurement Using an Automated Differential Viscometer. LCGC August 2024; Volume 20; Issue 8; Pages: 23–27
7. ISO 1628-5:1998; Plastics-Determination of the Viscosity of Polymers in Dilute Solution Using Capillary Viscometers Part 5: Thermoplastic Polyester (TP) Homopolymers and Copolymers.
8. ISO 1628-1:2024 Plastics – Determination of the viscosity of polymers in dilute solution using capillary viscometers – Part 1: General Principles

This application note was developed in collaboration with Lotte Chemical (Seoul, South Korea), which has supplied the samples and some results for this study.