Hello and welcome to a new rule of thumb post. Today we discuss how to select the optimal PET grade (fiber vs sheet vs bottle grade) for your application.
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| Figure 1: Selecting the optimal PET grade |
PET Intrinsic Viscosity (IV; dL/g) as a Key Indicator for selection and quality
Polyethylene terephthalate, or PET, is a ubiquitous polymer in our daily lives, from clothing to packaging. John Rex Whinfield and James Tennant Dickson, British chemists, are credited with inventing polyethylene terephthalate (PET) in 1941. They patented the material while working at the Calico Printers' Association of Manchester, building upon early research by Wallace Carothers. By using an aromatic diacid, properties of the resulting Polyester could be improved. This was the major start of PET which became the foundation for synthetic fibers like polyester, Dacron, and Terylene, as well as bottles, and sheets.
But not all PET is created equal. The key to its diverse applications lies in its Intrinsic Viscosity (IV), a crucial measure of its molecular weight and, consequently, its strength and processing characteristics. This is especially vital when utilizing recycled PET. Figure 2 shows the different process steps in PET production, including the IV obtained at every stage and the resulting potential applications.
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| Figure 2: PET process technology and the importance of Intrinsic Viscosity (IV) [4]. |
Application-Specific IV Ranges
For yarn and fiber applications, where flexibility and drape are paramount, PET with an IV between 0.6 and 0.7 dL/g is preferred. Moving up the scale, 0.7 to 0.8 dL/g PET is ideal for sheet applications, offering the necessary rigidity for thermoforming.
The familiar PET bottles we see daily, typically created through injection moulding into preforms, require an IV of 0.8 to 0.85 dL/g to ensure robust performance. For demanding uses like water jerry cans, where extreme durability is essential, PET with an IV greater than 0.85 dL/g is selected.
Summary of the PET IV ranges:
- Yarn & Fiber (0.6-0.7 dL/g)
- Sheet (0.7-0.8 dL/g)
- Bottle (0.8-0.85 dL/g)
- Jerry Cans (> 0.85 dL/g)
Choosing the correct IV grade is fundamental for achieving optimal product performance and ensuring efficient processing, highlighting why precise PET grade selection is critical for quality and sustainability across industries.
You would like to optimize your PET bottle - check out his post: Carbonated PET Bottles - Saving Material by Optimization Calculation
More Rule of Thumb posts can be found under "Start here"
Literature:
[1] https://cirplus.com/learn-and-explore/IV-measurement-for-rpet
[2] https://specialtypolymers.indoramaventures.com/
[3] https://www.alpla.com/en/products-innovations/innovations/pet-wine-bottle
[4] Nexant - the global petrochemical industry, 2017.
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