Technology Spotlight: Episode 12
What’s the difference between liquid & solid IV lifters for rPET? Let’s find out.
For recyclers and converters of Polyethylene terephthalate (PET), improved processing and greater throughput are consistent challenges on the path toward a circular economy. Degradation of molecular weight from oxidative damage, in-service use, heat history, and the extrusion process itself, continues to defy those involved with the upcycling of this most-recyclable plastic in the world [1].
A solution has emerged in the form of reactive additives that can rebuild molecular weight of rPET via chain extension and mild branching. The heat and residence time of the extrusion process was found to be sufficient to enable the reaction between active ingredient and pendant hydroxyl and carboxyl groups. As measured by intrinsic viscosity (IV), levels can be restored to those of virgin PET, i.e. 0.75 - 0.85 dl/g [2], and more if desired. Higher IV ensures more efficient processing in the extruder and greater dimensional stability of fabricated sheets, bottles, foams, etc.
These reactive additives may be based on various chemistries. A previous post [3] covered these in greater detail.
Today’s focus is on dianhydrides as the active ingredient, but here’s the twist. Dianhydrides, particularly Pyromellitic dianhydride (PMDA), are solids, often supplied as fine or micronized powders. How can a powder be added to an extruder with any degree of efficiency? Ever bake a cake? Know what I mean? Due to the very low addition rate needed to manage the reaction between PMDA and rPET, metering becomes critical.
Two forms of reactive masterbatches are commercially available to address these issues.
One is in liquid form. How’s that, you say? Yes, a liquid, in this case, formulated suspensions or dispersions of micronized PMDA in a liquid vehicle. Sounds great to facilitate dosing, but hold on - there are storage, handling, and performance issues inherent to liquid systems, so let’s understand them together:
The nature of the liquid component needs to be defined for compatibility with rPET. It may comprise water, alcohols, reactive diluents, or the like, low enough in viscosity to permit significant loading of PMDA, but high enough to ensure adequate mixing with rPET during extrusion. This liquid phase may flash during the extrusion process, act as a lubricant, or end up somewhere in the finished product. Residual content should be verified.
These formulations require careful development to maintain physical stability over their useful life. This may involve surfactants and other additives. These also need to be accounted for in the extrusion process and end product. Additionally, formulations may break, particularly at higher temperatures or extended storage periods, affecting mixing efficiency. Shelf life of 6-12 months is typical.
Cost may be a consideration, as there may be ingredients in the formulation that impact cost/kg or cost-in-use. Consider what you’re paying for, and what you don’t necessarily want in your finished product.
Specialized dosing/metering equipment may be required, again impacting cost.
The other is in solid form. But we already established that PMDA is a solid! Yes, but reactive masterbatches are available in granule or pellet form. What’s so special about these?
PMDA is encapsulated in a solid carrier, ensuring consistent assay as the masterbatch is charged to the extruder. Masterbatches of different PMDA concentrations enable more precise metering, tailored to the specifics of your process equipment, often at a lower cost position.
PMDA assay is not compromised during the masterbatch manufacturing process, further ensuring quality.
The solid carrier is typically PET, so two ingredients in total. How much more compatible can you get than that? PET grades with different IV levels can be specified, similar to the composition of the rPET feedstock.
Masterbatches are supplied dry and do not require pretreatment or conditioning prior to use.
They can be added via existing side feeders, hoppers, etc., so no need for new investments.
Solid masterbatches have an extended shelf life as long as the product is stored away from heat and moisture. Special packaging goes a long way toward promoting stability, often in excess of two years.
In the final analysis, either product may be right for the individual requirements of your process or asset train, so please direct all questions to your supplier prior to any lab evaluations, pilot trials or commercial runs.
Want to learn more about reactive masterbatches for polyester and polyolefin upcycling? Reach out today for an initial consultation.