Should we be using phthalate-free polypropylene? (TIA)

With the ongoing Covid-19 pandemic, people are getting more familiar with products made of polypropylene resin, such as the ubiquitous face masks and omnipresent takeout food containers.

Minghui Zhang
Evonik Catalysts

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Article Summary

However, polypropylene has been gaining popularity ever since it was introduced to the world in the early 1950s. It is now the second most used plastic across the globe, just behind its close cousin, polyethylene. In 2020 polyethylene consumption was about 108 million metric tons and propylene was about 76 million metric tons according to IHS Markit.

Both polyethylene and polypropylene are produced commercially by polymerising their respective small molecular monomers, ethylene and propylene, with the assistance of a family of catalysts known as Ziegler-Natta catalyst. Due to its slightly more complex molecular structure, different versions of polypropylene exist. To selectively produce the useful grades of polypropylene, a promoter is incorporated into the Ziegler-Natta catalyst. Various organic compounds have been tested and used as the catalyst promoter, also known as the internal donor, for propylene polymerisation.

New generation
Phthalates, the esters of phthalic acid, have been widely used in polymers as a plasticiser to make the otherwise brittle PVC into more flexible and durable plastic pipes and sheets for almost 100 years. In the 1980s, scientists found they could also do wonders as the internal donor for polypropylene catalysts.

When phthalate was incorporated in the catalyst, a so-called fourth generation of Ziegler-Natta was invented. This generation increased catalyst activity by almost 10 times and selectivity by 10% over the original version of Ziegler-Natta catalyst. This combination eliminated the requirement for a post-treatment process in polypropylene production to get rid of undesired grades of polypropylene and metal as well as chloride residues carried over from the catalyst. The fourth generation of catalyst shortened the production process, significantly increased raw material utilisation, and reduced energy consumption as well as waste generation.

However, due to the long and widespread application, and the fact they are easily released from PVC articles, phthalates have been scrutinised recently. They are believed to be harmful to the environment and health in high concentrations. In the case of polypropylene, the negative impacts of phthalates are less clear. One reason for the lack of clarity is that it is difficult to detect phthalates in polypropylene, even though it is not removed at any stage of the production process. Some argue that this means it is destroyed during processing, but this hypothesis is also unproven. Whatever theory you lean towards, it is becoming apparent that phthalate-free polypropylene is increasingly attractive.

What are the alternatives?
Evonik Catalysts includes a phthalate-free option in its range of Catylen products. Catylen is Evonik's brand of olefin polymerisation catalyst components and comprises Catylen S (supports) and Catylen D (donors), thus offering a specialised product portfolio for the manufacture of polyolefin catalysts. These products are proprietary to Evonik, produced on a commercial scale and globally available.

Catylen D 2100 is a phthalate-free and non-aromatic internal donor for Ziegler-Natta catalysts used in propylene polymerisation processes, enabling customers to exceed the highest toxicity standards.

This commercial diether compound is used as a promoter in a similar way to phthalates but with better results, with between 25% and 100% improvement in activity and further enhanced product selectivity. It is highly pure and especially suitable for high-impact copolymer applications, like injection moulding parts for automotive, one of the fastest growing polypropylene applications. It has been proven to increase catalyst activity and hydrogen response and provide narrow molecular weight distribution.

Polypropylene plays an essential role in modern life, and we should do all we can to ensure its production does not have negative side effects. Choosing the best catalyst options available can help.

This short case study originally appeared in PTQ's Technology In Action Feature - Q1 2023 Issue.

For more information: Minghui.Zhang@evonik.com

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