PET presents many advantages including aesthetics, 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. Control, or lack thereof, of PETs intrinsic viscosity at the extrusion stage can make or break packaging performance, resulting in issues ranging from downstream quality problems to added cost.
Continue reading to learn more about the importance of intrinsic viscosity and ICPG’s quality control capabilities:
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 of the monomers with water as the 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 related to its end use application. This is measured using an intrinsic viscosity, or IV, testing technique.
Why Intrinsic Viscosity Matters in PET Grade
|
PET grade |
Intrinsic Viscosity [dL/g] |
|
Fiber grade |
0.40 – 0.70 |
|
Film grade |
0.70 – 1.00 |
|
Bottle grade |
0.70 – 0.78 |
|
Water and soft drink bottle grade |
0.78 – 0.85 |
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 a certain level for its end-use application based on its IV test reading.
A measure of the polymers molecular weight, IV reflects the material’s melting point, crystallinity, and tensile strength. Intrinsic Viscosity is, thus, a key quality control specification and perhaps the most important characteristic of PET.
The IV is dependent upon the length of the polymer chains. Therefore, the longer the polymer chains are, the more entanglements between the polymeric chains occur, and the higher the viscosity value resulting in a stiffer material.
The desired IV of PET is dependent on its application and classified in different grades as referenced in the table.
Advantages of Testing and Controlling Intrinsic Viscosity
Testing and controlling intrinsic viscosity are crucial for PET's processability and overall quality of the product.
Processing low or unknown IV grade PET can lead to undesired effects, such as extrusion pressure and melt viscosity variation, resulting in sheet thickness variation or brittleness. The same follows at the thermoforming process stage with high deviation in the process and scrap rate.
PET inherently presents challenges in processing, and this can be further exacerbated with the integration of recycled PET, or RPET.
RPET changes the intrinsic viscosity of the material, as the metallic catalysts used in the initial polymerization loses its strength and cannot be revived. This results in a loss of IV, leading to downstream issues, particularly now that sustainability has become a pressing issue and the integration of RPET anywhere from 10 - 100% has become a must.
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 before the PET can be extruded. 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 affects the material’s performance, but also the bottom line in terms of unwanted scrap, waste, additional process runs and related costs for both the extruder and thermoformer.
Improving Quality Through Controlled Intrinsic Viscosity in Extrusion Processes
The mechanical characteristics of PET improve at higher molecular weight (higher IV) and are worse at lower molecular weight (lower IV). Should the IV of the extruded PET sheets drop below acceptable thresholds, the end product’s mechanical properties of the sheet will be compromised to an unacceptable level, resulting in downstream quality issues in thermoforming processes.
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.
This is where real-time IV monitoring during the extrusion process comes into play. Impact Plastics’ new state-of-the-art PET sheet extrusion line has the ability to monitor and control the relative level of the material IV in-process and in real-time.
ICPG's real-time IV monitoring identifies any deviation in processing conditions and is able to take corrective action to address those conditions to ensure consistent quality and an efficient production run. These results are benchmarked against the ASTM Solution Test Method for measuring intrinsic viscosity and possesses 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 and have consistently shown this material to offer an excellent sustainability footprint for packaging applications. In addition, the safety and use of PET have been repeatedly demonstrated through extensive research, scientific test studies, and regulatory approvals for its use and reuse.
Now, there are more opportunities than ever to use recycled PET in packaging applications. 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 integration provides an additional means of improving as a means to reduce the amount of virgin plastics used in packaging solutions.
Technological advancements in PET extrusion technology and the use use of real-time IV quality monitoring processes ensure that intrinsic viscosity specifications are met without sacrificing quality, thereby supporting circular economy practices.
Interested in launching your next PET packaging project with ICPG? Get in touch to learn more about our capabilities customization options:
