Technology Spotlight: Episode 10

Feb 5

Understanding the chemistry of reactive IV enhancers for rPET processing.

Polyethylene terephthalate (PET) finds use in flexible and rigid packaging, synthetic fiber for clothing, and yarn for carpet applications. The life cycle of PET, from the processing of prime resins, to molded articles in service, to recycling of waste plastic, results in continuous polymer degradation, as evidenced by oxidative color change, molecular weight reduction, and acetaldehyde formation. The primary metric for PET processability, intrinsic viscosity (IV), decreases considerably from bottle grade rPET, typically at 0.80-0.82 dl/g, to lesser quality feedstocks, as low as 0.5 dl/g.

Interestingly, carboxyl and hydroxyl groups formed during PET degradation can be leveraged for the recycling process, as they react with various additives to rebuild molecular weight, increase IV, improve processability and restore mechanical properties.

This begs many questions, so let’s begin:

What is reactive extrusion?

Reactive extrusion involves the use of additives that react chemically with waste plastics within the residence time and temperature constraints of the extrusion process, specifically to improve flow, dimensional stability and throughput. These may be single active ingredients or mixtures in masterbatch form.

Are all additives reactive?

In a word, no. Surfactant-type additives are sometimes included in this category, but these are external processing aids i.e. surface-active-agents. While they may have affinity for certain functional groups, to influence flow behavior, recyclate content, or weatherability in-service, they do not chemically react with rPET.

Another example is PET itself, added as prime resin to compensate for the compromised rheology of rPET. Loading level can vary depending on the quality of the rPET feedstock, so the proper cost-material balance needs to be established. Emerging regulatory requirements on the recycled content of PET articles, for example 30% in the EU and 50% in California by 2030, may preclude this option.

What are the classes of reactive additives?

First up are dianhydrides. These include Pyromellitic dianhydride (PMDA), although others may be selected for certain applications. PMDA is a compact, tetrafunctional molecule, capable of chain extension, branching, and crosslinking. For more on PMDA and its uses, read Episodes 4 and 9.

Next are glycidyl methacrylate (GMA) copolymers. There are several variations of GMAs including oligomers with styrene and acrylonitrile blocks. These macromolecules feature pendant epoxide groups that also engage in chain extension, branching, and crosslinking.

A third example is hybrid systems e.g. block copolymers capped with anhydrides, or physical blends of anhydrides and GMAs. These are cited in the literature and typically custom-synthesized.

A fourth example is bisoxazolines. These are known acid scavengers, and in the case of rPET, can engage in chain extension, albeit with minimal effect on IV. Bisoxazolines are typically used to fine-tune rPET processing.

Are there pros and cons to these additives?

Dianhydrides require minimal dosing to achieve the desired increase in IV. For example, at low levels, selective reaction with rPET hydroxyl groups occurs, resulting in singular rPET-PMDA and dual rPET-PMDA coupling. Branching and crosslinking occur when idle carboxylic groups on PMDA react with rPET, generating water. Extensive crosslinking, leading to gelation, may be caused by overdosing. The main takeaway is that less is best.

Since PMDA dosing levels require careful control, dilute masterbatches are the most convenient physical form to accurately charge extruders, typically at 0.5-3% loading levels. They are also the most effective means to tackle significant IV swings, in some cases from 0.5-1.2 dl/g, due to feedstock quality.

GMAs in 100% active form require minute, typical 0.1-1.0% dosing levels to modulate IV and molecular weight build within a limited range. Liquid metering of molten GMAs may be a better option. Due to the reactivity of epoxides with both hydroxyl and carboxyl groups, the opportunity for rPET crosslinking exists, as evidenced by marked increases in IV, impeded flow and gelation.

Keep in mind that rPET degradation occurs during drying and extrusion processes, demanding even more from additives to compensate for resultant IV losses.

Additives require compliance with EFSA and FDA regulations for incidental food contact, so commercial products are typically best. Active ingredients typically have maximum allowable levels to maintain compliance, and this should also be confirmed prior to selection.

What about crystallinity?

By definition, molecular weight build from chain extension and branching creates disorder and hinders crystallization, so the use of additives can decrease crystallinity and improve clarity.

How do additives influence mechanical properties?

Mechanical property improvements with dianhydrides are limited to tensile strength and modulus, due to their aromaticity and rigidity. The block copolymer structure of GMAs introduces a degree of toughness, addressing impact resistance and elongation. However, due to the many qualities and inconsistencies of rPET feedstocks, your mileage may vary.

What about cost?

Due to the wide use of dianhydrides in many types of chemistries and end applications, global capacity is significant, and availability is widespread. As a result, dianhydride-based additives tend to be lower in cost than GMA-based additives. In all cases, an economic analysis is recommended to encompass all potential cost levers including materials, process efficiency and throughput, and energy demand.

Want to learn more about how reactive masterbatches can be leveraged to optimize your rPET process? Reach out for a free initial consultation today.

References

https://www.nexamchemical.com/products/reactive-recycling/iv-enhancers-for-pet/

https://patentimages.storage.googleapis.com/ee/ec/49/fac9c2fda2149b/US20120016090A1.pdf

https://azelisamericascase.com/wp-content/uploads/2020/11/Joncryl-ADR-4368-tds.pdf

https://www.techmerpm.com/wp-content/uploads/2023/02/HiTerra-rPET-Revive-One-Pager-Final.pdf

https://www.polyvel.com/polyvel-introduces-rboost-pet-chain-extender/

https://www.silmaster.com/en/silmalink/

https://content.yudu.com/web/1rl19/0A1rl2p/CWOct23/html/index.html?origin=reader

https://www.shepherdcolor.com/wp-content/uploads/2021/04/PIP_April2021_PursuitPurity-article.pdf

https://www.hollandcolours.com/markets/packaging/issues-facing-the-rpet-plastic-market#stuk1

https://www.mdpi.com/1996-1073/14/21/7306