Technology Spotlight: Episode 8

Solid State Polymerization, Liquid State Polymerization, and Reactive Recycling: A promising collaboration for rPET processing.

Industry solutions for recycled Polyethylene terephthalate (rPET) processing, sustainability and circularity have increased in complexity and performance in recent years. Austria has become a hotbed of activity for rPET turnkey processing solutions. Starlinger, Erema, NGR, and others offer innovative designs and robust support programs.

But let’s back up a bit. The legacy technology, solid state polymerization (SSP), is based on a reactor design to rebuild molecular weight with heat and pressure in a nitrogen and/or vacuum environment. SSP is frequently used to increase intrinsic viscosity (IV) and decontaminate rPET pellet feedstocks prior to injection blow molding or extruding.

SSP has a significant energy demand, requiring heating to just below the glass transition temperature, then to just below the melt temperature. To address energy and time demand, liquid state polymerization (LSP) is promoted as an alternative. Essentially an inline polycondensation unit, the LSP reactor, charged directly by the extruder, processes rPET in the melt phase, improving IV and impurity content within minutes, typically below the 0.30 kWh/kg benchmark for energy efficiency (1).

Newer SSP designs involve placement of the reactor upstream to dry, decontaminate, and increase the IV of rPET flake feedstocks prior to entering the extruder intake. This also serves to minimize decomposition during the extrusion process (2). 

These variations all involve considerable dwell time in the reactors, regardless of design. Up to 30 minutes for LSP and several hours for SSP need to be considered for adequate processing and overall cycle time (3,4). 

So while there are great gains to be realized from incorporating these assets into your rPET process train, how can energy demand, cycle time and IV limitations be further managed for greater efficiency and higher throughput?

Consider Reactive Recycling technology from Nexam Chemical AB as the ideal complement to LSP and SSP processes. Reactive Recycling involves the reactive extrusion of rPET with thermally active masterbatches in granule form. IV increase is possible over a wider range, typically from 0.5 to 1.2 dl/g, based only on the dwell time in the extruder, without additional equipment or process modifications. Reactive masterbatches have proven their effectiveness for IV enhancement of rPET across a wide variety of packaging, fiber, and foam applications.

In this case, Reactive Recycling can be thought of as a polishing step for LSP and SSP processes, allowing for

• greater IV increases

• reduced cycle times

• lower energy consumption

• further optimization

all at the best possible cost position.  

Reactive Recycling also enables a wider selection of rPET feedstocks, moving beyond bottle-to-bottle recycling.  Here are some examples (ref. internal data):

Want to learn more about how LSP, SSP and Reactive Recycling can be leveraged to optimize your rPET process? Reach out for a free initial consultation today.

[Note: In some industry references to LSP and SSP, P may denote Polycondensation rather than Polymerization.]