Rapid Quality Control in PET Production

Rapid Quality Control in PET Production: Analysis of Diethylene Glycol, Isophthalic Acid, Intrinsic Viscosity and Acid Number in Under One Minute Using Vis-NIRS

Introduction

Plastics and polymers have become an integral part of modern life today. Based on their physical structure, the variable parameters they contain and their different degrees of flexibility, they can be transformed into virtually any shape. The manufacturing processes of plastics may vary according to their intended use, and today they can be found in a wide range of products spanning from aircraft and automotive parts to packaging, medical devices to textiles (and many other components vital to sustaining our lives).

Polyethylene Terephthalate (PET)

PET (C10H8O4)n is a thermoplastic polymer belonging to the polyester family that is widely used especially as PET bottles and food packaging material frequently encountered in our daily lives. Polyester resins are known as a flawless combination of properties such as mechanical, thermal and chemical resistance alongside dimensional stability. [caption id="attachment_131235" align="aligncenter"] Figure 1: Molecular structure of linear PET.[/caption] In addition to linear PET, a branched form of the polymer also exists. Branched PET is typically blended with a small percentage of isophthalic acid (C8H6O4). Purified isophthalic acid (PIA) reduces the crystallinity of PET, helps improve clarity and increases the efficiency of bottle production processes. Another additive, diethylene glycol (DEG), reduces the crystallization rate of PET isothermally and dynamically during crystallization from the molten form. [caption id="attachment_131236" align="aligncenter"] Figure 2: Molecular structure of isophthalic acid.[/caption] PET in its natural form is a fairly flexible, colorless and semi-crystalline resin. Depending on the processing method, it can be semi-rigid or rigid. It has resistance to impact, moisture, alcohols and solvents. PET is frequently used in numerous industries such as food, textiles and automotive due to its fundamental properties and the numerous advantages it provides. In particular, due to its strength and lightweight, it frequently appears as a food packaging material. Its safe use in the food and beverage sectors has been approved by the FDA, EFSA, Health Canada and other health authorities. In addition to all the advantages it offers, PET is one of the most recycled thermoplastics and carries the number 1 recycling symbol. Recycled PET can be converted into fibers, fabrics and sheets for packaging and automotive parts production. Approximately 60 percent of global PET production is used in textile fiber manufacturing, while approximately 30 percent is used in bottle production for various purposes. The recyclability feature is particularly attractive for manufacturers who want to save costs and operate in a more environmentally friendly manner. Various standard analytical techniques and methodologies are used for parameters monitored in PET quality control processes; HPLC/HPLC-MS analyses for diethylene glycol and isophthalic acid measurements, viscometric measurements for intrinsic viscosity and titrimetric determination for acid number are among the most prominent. In addition to the long and difficult sample preparation stages that many of these techniques require, potential errors that may occur in these processes can cause significant delays in production stages and, depending on faulty production, financial losses reaching up to 30 percent of total revenue. Within this review article, we aim to examine the analyses performed in PET production quality control and provide detailed information about the advantages that Near-Infrared Spectroscopy (NIRS) techniques can provide to these processes.

Near-Infrared Spectroscopy (NIRS)

Near-infrared spectroscopy (NIRS) is a technique based on the principle of molecule-light interaction and provides significant contributions towards the practical, rapid and precise monitoring of critical parameters in quality control analyses compared to conventional methods. In addition to being a non-destructive method, NIRS technique allows you to perform faster, more environmentally friendly and more cost-effective analyses compared to wet chemistry methods thanks to the fact that it does not require any sample preparation. A NIR spectrometer measures the interaction between light and matter to produce a spectrum. Near-infrared spectroscopy is particularly sensitive to the presence of specific functional groups such as -CH, -NH, -OH and -SH. This makes NIR spectroscopy an ideal method for measuring many chemical parameters such as water content (moisture), hydroxyl value, acid number and amine content. In addition to chemical parameters, since the interaction between matter and light also depends on the matrix of the sample, it is also possible to determine physical and rheological parameters such as density, intrinsic viscosity and melt flow rate.

Quality Control Points with NIRS in the PET Production Chain

PET quality control analyses are important and necessary at specific many steps in the process from the production of pure polymers to the processed final product form. Several steps in the production chain are optional (additives), typically extending from raw plastic producers to plastic parts producers, and the operation steps are typically as follows: [caption id="attachment_131237" align="aligncenter"] Figure 3: Quality control points with NIRS in the PET production process.[/caption] The first point where near-infrared laboratory instruments can be used is where pure polymers such as PET are produced and their purity needs to be verified. NIRS is also a very important and beneficial technique for the next operation where polymers are combined with additives to create intermediate products for later use in further steps. Typically, a plastic parts manufacturer with an injection molding or extrusion process evaluates the quality of the polymer batch received to achieve efficient production. In many cases, without verification, the certificate from the supplier is relied upon. On the other hand, the rapid increase in the number of companies producing for the medical industry or producing parts with high value or quantity has brought with it the need to monitor important rheological quality parameters for each polymer batch before use in injection molding or extrusion processes. Feeding a polymer outside of specification into the production process leads to expensive equipment downtime as well as time-consuming cleaning processes. Rapid pre-control of the initial polymer materials used in the process is an ideal solution to prevent such risks and possible time losses. In this context, NIRS stands out with its high speed, low operating costs and usability by personnel without detailed chemistry knowledge. When the final part is formed at the end of the production process, quality control analyses can also be performed again using NIRS. (e.g. evaluation of the homogeneity or thickness of bottles or material layers) It is very important to control certain parameters to ensure quality product output during PET production. These quality parameters typically include diethylene glycol content, isophthalic acid content, intrinsic viscosity (ASTM D4603) and acid number (AN). Due to the limited solubility of PET, the determination of parameters listed in Table 1 due to the need to use different analytical methods and their associated various measurement equipment can involve long and difficult processes with conventional laboratory techniques.

Table 1: Result times for parameters with conventional techniques.

With Metrohm's new Polymer Analyzer system that combines visible (Vis) and near-infrared (NIR) regions, it is possible to measure various quality control parameters both chemical and physical in under one minute without any sample preparation, non-destructively, without chemicals and without the need for consumables, using techniques developed in accordance with ASTM E1655-17 NIRS application guidelines.

Experimental Content and Workflow

Within our review, using a Metrohm Polymer Analyzer, repeated measurements were performed on PET pellets in reflection mode across the entire Vis-NIR wavelength range (400–2500 nm). [caption id="attachment_131239" align="aligncenter"] Figure 4: Metrohm Polymer Analyzer (Solids).[/caption]   PET pellets were placed in a DS2500 Large Sample Cup rotating sample container and thus repeatable spectrum scan assurance was ensured by taking fully automatic measurements from different points, eliminating potential factors depending on particle size or chemical component irregularities. As can be seen in Figure 5, all measurements were performed without any sample preparation. [caption id="attachment_131241" align="aligncenter"] Figure 5: Metrohm Polymer Analyzer (Solids), image of PET pellets on the rotating DS2500
Large Sample Cup.[/caption]

Method Development

The Vision Air Complete software package was used for data collection, model building and quantification method development processes. Through this triple package software that takes the user interface experience to a whole new level, all instrument management including the assignment of operation procedures is applied from within the office with the help of Vision Air Manager Network, while all actual measurements are performed in the quality control laboratory with the Vision Air Routine interface. The obtained spectra were automatically uploaded to the database and made directly accessible for method development. Quantitative method development was carried out using Vision Chemometrics Software.

Analysis Results

The obtained Vis-NIR spectra (see Figure 6) were used to create prediction models for quantitative determination of diethylene glycol, isophthalic acid, intrinsic viscosity and acid number parameters. [caption id="attachment_131242" align="aligncenter"] Figure 6: Selection of obtained PET Vis-NIR spectra. (Offset has been applied to spectra due to image adjustments.)[/caption] The quality of the prediction models was evaluated through correlation graphs showing the relationship between values from primary methods (titration, viscometer, HPLC and HPLC-MS) and Vis-NIR data (see Figures 7, 8, 9 and 10). Reliability data (FOM) regarding the expected data accuracy during routine analysis is also shown in these figures. [caption id="attachment_131243" align="aligncenter"] Figure 7: Vis-NIR correlation graph and reliability values for diethylene glycol content in PET samples. (Diethylene glycol laboratory values were evaluated using HPLC-MS).[/caption]   [caption id="attachment_131244" align="aligncenter"] Figure 8: Vis-NIR correlation graph and reliability values for isophthalic acid content in PET samples. (Isophthalic acid laboratory values were evaluated using HPLC).[/caption] [caption id="attachment_131245" align="aligncenter"] Figure 9: Vis-NIR correlation graph and reliability values for intrinsic viscosity in PET samples. (Intrinsic viscosity laboratory values were evaluated using a viscometer).[/caption] [caption id="attachment_131246" align="aligncenter"] Figure 10: Vis-NIR correlation graph and reliability values for acid number in PET samples. (Acid number laboratory values were evaluated using titration method).[/caption]

Conclusion

This review demonstrates that typical key quality control parameter analyses in PET samples can be performed using Near-Infrared Spectroscopy (NIRS) technique. Compared to wet chemistry methods, rapid and simultaneous analyses highlight the most striking advantage of NIRS technique. The ability to determine all parameters (both physical and chemical) in under one minute within a single measurement makes it possible to achieve superior time and cost savings especially in the polymer industry. As Metrohm, in addition to being a global sector leader in primary methodologies (potentiometric titration and Karl Fischer titration) that form the basis of NIRS analysis techniques in the polymer industry, we directly offer fully integrated, rapid, precise and reliable solutions with our Vis-NIRS system solutions and publications under the same roof that comply with international standards.

Sources / References

1. Metrohm Application Note, AN-NIR-023 Quality Control of PET

2. Metrohm Brochure, 80005287 Polymer and plastics analysis – Quality control for polymer production

3. Metrohm Blog, NIR Spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3, https://metrohm.blog/2021/06/14/nirs-qc-polymers-part-3/

4. Metrohm Blog, NIR Spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1, https://metrohm.blog/2021/05/03/nirs-qc-polymers-part-1/

   

Şahin Yalçın Sales and Application Engineer Process Analytics / Spectroscopy Metrohm Turkey