Why Intrinsic Viscosity Matters in PET Packaging

Polyethylene Terephthalate (PET) presents many advantages, including aesthetics, visual appeal, strong barrier properties, durability, recyclability and sustainability, thus making it the material of choice for a wide range of packaging applications across various market segments.

A key specification in PET that drastically affects its functionality and processability in both extrusion and thermoforming is the intrinsic viscosity. Effective control, or lack thereof, at the extrusion stage can significantly influence packaging performance, resulting in issues ranging from quality defects to increased operational costs.

In this article, we explore the importance of intrinsic viscosity and how ICPG’s advanced quality control capabilities ensure precision and reliability in PET processing.

Understanding PET and Intrinsic Viscosity

Most of us can probably picture what rigid PET packaging looks like in its end-use application, but how is it made?

PET is produced by copolymerizing monomers including Ethylene Glycol and Terephthalic Acid under heat and pressure with the help of metallic catalysts.

Polymerization occurs through a polycondensation reaction, producing water as a by-product. As the viscosity increases during the polymerization stage, so does the polymer chain and chain length.

The reaction is stopped at the desired polymer chain length which is determined by the intended end-use application. This is measured using an intrinsic viscosity (IV) test, which quantifies the average molecular weight and polymer chain length.

Why Intrinsic Viscosity Matters in PET Grade

You have probably heard of the term IV when discussing PET materials, but why is this specification so important?

As previously mentioned, the polymer chain reaction, or polymerization, is stopped at a point when polymer chain length reaches the required level for its application based on intrinsic viscosity test results.

A measure of the polymers molecular weight, IV reflects critical characteristics such as the material’s melting point, crystallinity, and tensile strength. It is a key quality control specification and perhaps the most important characteristic of PET.

Since IV depends on polymer chain length, longer chains lead to more entanglements and a higher intrinsic viscosity, resulting in a stiffer material.

The desired IV level depends on the PET’s application, which is why it is categorized into different grades, as shown in the table.

Advantages of Testing and Controlling Intrinsic Viscosity

Consistent testing and precise control of intrinsic viscosity are crucial for PET's processability, performance, and final product quality.

Processing PET with low or unknown IV grade can lead to issues such as variation in extrusion pressure, unstable melt viscosity, and sheet thickness deviations, which may cause brittleness or structural defects.

These processing challenges become even more prominent during thermoforming, where poor IV control leads to higher scrap rates and production inefficiencies.

PET also presents additional challenges when integrating recycled PET (rPET). The intrinsic viscosity of rPET is typically lower because metallic catalysts degrade during initial polymerization process and cannot be reactivated. This results in a reduction in IV, making consistent processing more difficult.

Today, sustainability demands the use of rPET at levels from 10% to 100%, which further emphasizes the need for reliable IV control.
Additional factors affecting the intrinsic viscosity are drying and processing parameters. PET is hygroscopic, meaning it absorbs moisture very easily, and it must be thoroughly dried prior to extrusion. When half-dried or wet PET or rPET is processed, it could result in a product with low IV due to its moisture content.

As such, the economics of using an unsatisfactory and unknown IV of PET material are significant. Ineffective management of intrinsic viscosity not only compromises performance, but also drives up costs through increased scrap, waste, reprocessing, and production delays.

Improving Quality Through Controlled Intrinsic Viscosity in Extrusion

PET’s mechanical properties improve at higher molecular weight (higher IV) and worsen at lower molecular weight (lower IV). If the IV of the extruded PET sheets drop below acceptable thresholds, mechanical strength, clarity, and forming characteristics suffer, leading to downstream quality issues during thermoforming.

Most extrusion processors of PET require outside sources and labs to perform IV testing, which can take days to weeks to get the results. Most likely, by the time the material’s intrinsic viscosity is known, the quality issue has already been discovered and the material rejected.

ICPG’s new, state-of-the-art PET sheet extrusion line includes integrated, real-time intrinsic viscosity monitoring technology. This system can measure and control IV levels during the extrusion process, ensuring quality assurance without delay.

This proprietary system detects any deviation in processing parameters and enables corrective action in-process, minimizing waste and maximizing efficiency. The data is benchmarked against the ASTM Solution Test Method for measuring intrinsic viscosity, delivering an accuracy of +/- 0.02 dl/g.

Enhancing Circular Economy Practices with Advanced Extrusion Technology

The global demand for more sustainable packaging solutions has put the issue of sustainability and recyclability top-of-mind in the plastics industry.

The closed-loop recycling of used PET bottles into new food-grade PET containers presents a significant opportunity to extend the environmental benefits and sustainability of PET as a packaging material.

Extensive life-cycle analysis studies have examined PET’s energy use and greenhouse gas emissions, consistently showing it offers an excellent sustainability footprint for packaging applications. In addition, the safety and reliability of PET have been repeatedly demonstrated through extensive research, scientific test studies, and regulatory approvals for both its use and reuse.

Today, manufacturers have more opportunities than ever to incorporate recycled PET into their packaging strategies. The infrastructure established over the past decade has made it possible to produce Post-Consumer Recycled (PCR) PET solutions that meet the same quality and safety standards as virgin PET. In addition, the use of post-industrial regrind provides another effective way to reduce reliance on virgin plastics.

ICPG’s advanced extrusion technology, combined with real-time intrinsic viscosity monitoring, ensures that sustainability goals can be met without compromising product quality.

Ready to Improve Your PET Packaging Performance?

Looking to optimize quality, reduce waste, and support sustainability with PET packaging? Let ICPG help you leverage the power of intrinsic viscosity control in your next PET packaging project. Get in touch with our team to discuss your goals and explore our custom solutions.