Technology Spotlight: Episode 11
April Fools’ Day jokes aside, it’s time to start believing in the promise of Polypropylene circularity.
The holy grail of plastics technology, circularity, will be found once the proven reuse of waste plastic, sustainably recycled into renewable bottles, packaging, grocery bags, clothing, footwear, etc., is realized.
Some plastics, such as Polyethylene terephthalate (PET), are making significant strides toward circularity, as waste materials are relatively homogenous, clean, and well-understood in terms of upcycling. No wonder it’s recycling symbol number 1.
How about the other, more challenging plastics, such as polyethyene (PE) and polypropylene (PP)? These are admittedly more difficult due to the quality of post-consumer-recycled (PCR) and post-industrial-recycled (PIR) feedstocks, and the expectation that mechanical properties can somehow be restored to match their virgin analogs.
Consider the following:
PP molded parts in regular service may be exposed to thermal, abrasive, and chemical environments that induce oxidation and the formation of new functional groups along polymer chains.
Mechanical recycling of rPP results in further deterioration and loss of molecular weight, leading to less efficient processing and properties.
Degradation via repeated extrusion cycles induces chain scission, resulting in a truly downcycled polymer, a prime candidate for additive technology.
While certain organic peroxides are marketed for polyolefin upcycling, a very different chemistry has been validated by recently published research conducted by Dr. Alberto Frache and his group at the Department of Applied Science and Technology, Politecnico di Torino, Italy.
Not only did Frache’s group demonstrate that the molecular weight of rPP can be selectively rebuilt using the new additive, offered as a reactive masterbatch for dosing efficiency to the extrusion process, it can lower the rate of viscosity decrease over repeated extrusions, analogous to multiple recycling of the polymer.
His work can be summarized in Figure 4, kindly reproduced from the report:
These data indicate that:
the loss in viscosity over nine repeated extrusions can be partially restored with the addition of 5 wt% reactive masterbatch (green symbol).
mitigation of viscosity loss to vPP occurs, as demonstrated in the first extrusion (star symbol).
The mild branching of rPP created with this chemistry may be beneficial to certain downstream processes e.g. blow molding.
In summary, a commercially available reactive masterbatch (NEXAMITE R201 from Nexam Chemical AB) has been proven to provide significant recovery of rPP molecular weight over repeated cycles at a viable cost position, offering promise for the circularity of this challenging polymer.
Interested in learning more about the upcycling of polyolefins and polyesters? Reach out today for an initial consultation.