RECYCLING: Compatibilizers Create New Recycle Feedstream Value

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Generally confined to prime material applications, these additives are helping recyclers improve yields of mixed-material bales.

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Compatibilizers have long been used as a tool for prime resins to create special blends that yield desired performance and properties that otherwise could be obtained only by a blend of materials that would normally not be compatible.

In layman’s terms, compatibilizer additives allow resins that would not neatly blend to “talk to each other” and bond in a way that creates enhanced performance when compared with either polymer alone. In injection molding, for example, incompatible polymers cannot be processed because they delaminate during melt processing, causing rejects. Also, some virgin polymers experience chain scission (“scissoring” of molecular chains) during melt processing, and the resultant recycle resin has poorer mechanical properties compared with the starting virgin resin due to lower molecular weight.

The use of compatibilizers is being explored increasingly in the recycling industry as a way to create value in mixed feed streams that cannot be further segregated by resin type, either due to technical challenges related to collecting, cleaning, and sorting, or to economic infeasibility.

THE OPPORTUNITY
Recyclers continue to see a decrease in bale quality and yields. The residual materials that are left over after a bale has been processed are often sold for a few pennies per pound (if they can be sold at all), which is far less that what the recyclers actually paid for the bale. Recent findings suggest HDPE recyclers are experiencing a 20% yield loss; in PET recycling the yield loss is 40%. This rate of material loss can quickly change the economics of an operation from black to red. Similarly, in bales of material where the resin types are inherently highly mixed, such as bales of plastics from electronics, the most desirable resin types like ABS and HIPS might make up only about 60% of the bale.

What is a recycler to do with the other 40% mixed fraction that might not be separable because of technical or economic infeasibility? If that yield loss could be put to use as another valuable feed stream, it could change dramatically the economics of an operation and further divert valuable plastics from the landfill.

In some cases recyclers are finding compatibilizers are key to recycling multi-resin products, such as flexible packaging. According to a presentation given by Dow Plastics at a recent conference, 50% of all flexible packaging on the market today is barrier packaging, meaning more than one type of resin is present. Barrier films often contain EVOH and/or nylon layers, which are incompatible with many polyolefins.

But by using compatibilizers, these layers of otherwise incompatible resins can be blended and then used in varying levels to make parts for durable goods, creating a true “up-cycling” opportunity. The goal is to eventually extend the demonstrated success of compatibilizers with mixed-resin, post-industrial streams to similar post-consumer streams of materials. Compatibilizers have the potential to create mechanical recycling end-of-life solutions for products such as drink pouches, coffee pods, and another types of packaging that are currently incompatible with today’s recycling systems. 

HOW COMPATIBILIZERS WORK 
There are three categories of compatibilizers available as material additives in the marketplace today. Those include:

 1.  Bipolar copolymer compatibilizers: A polymer may be incompatible with another polymer for a number of chemical reasons, such as their different polarities. Polymers with dissimilar polarities can be made compatible by using bipolar copolymer compatibilizers that bridge the polarities. For example, a number of commercial TPEs are based on block copolymers of polar aromatic styrene monomer and non-polar aliphatic butadiene monomer. When the butadiene styrene block copolymer is added to a resin blend, its built-in bipolarity acts to attract the dissimilar polar polymers, creating a compatibility effect. This approach works well with known segregated streams, such as a nonpolar polyolefin with a polar polymer such as nylon, but is of limited value in post-consumer recycle streams containing a multiplicity of polymers that vary from batch to batch of recycle.

 2.  Maleated copolymer compatibilizers: Bond formation between maleic anhydride-grafted polypropylene and nylon 6 by in-situ block-copolymer formation can be called reactive compatibilization. Maleated polymers can be prepared directly by polymerization or by modification during compounding via the reactive extrusion process. Their anhydride groups can react with amine, epoxy, and alcohol groups. For example, DuPont’s Fusabond M603 is a random ethylene copolymer, incorporating a monomer that is classified as a maleic anhydride equivalent for application uses.

Styrene maleic anhydride (SMA) copolymer is another suitable agent for compatibilizing normally incompatible polymers such as nylon/ABS blends. The limitation of this class of additives is their specificity, requiring known chemistry of the polymers to be compatibilized. In addition, maleic anhydride depolymerizes condensation polymers such as PET and PC, thus obviating its use in mixed streams such as PCR containing olefins, PET, and other assorted polymers.

 3.  In-situ macromolecular catalysts: Since monomers become polymers in the presence of catalysts, copolymerization of two or more dissimilar polymers in the melt via in-situ catalysis using thermally stable organometallics holds the possibility of allowing the use of high levels of PCR in consumer goods. Ester forms of organometallics such as titanates, zirconates, aluminates, and zirco-aluminates provide possible chemistries for R&D in using high levels of PCR in consumer goods, since they are already used as esterification catalysts for PET and as single-site metallocene catalysts for polyolefins. In addition, it is possible that this class of catalysts is synergistic with maleated copolymers.

The disadvantage of this class of additives is their use is not as yet well known to the recycling industry. Recyclers generally are not R&D chemists and often require simple additive systems and processes that can be replicated easily in a practical manner and at a reasonable cost. For example, recyclers will have to become more familiar with reactive compounding techniques to optimize the catalysis effects and will need the capability to monitor and adjust melt-process conditions to optimize the work energy (defined as the area under the plot of polymer melt torque vs. screw residence time).

THE CHALLENGES
Despite the opportunities that compatibilizers may offer in realizing new value in previously overlooked recycle streams, they also present some challenges. One of the most important is the generally inconsistent nature of the resin mix. It is often very hard to predict the exact mix of resins in any feed stream, let alone the residual content of that stream after sorting out the desired materials. This means some further processing and separation might be necessary, but the new potential recovery opportunity of that targeted fraction with compatibilizers might justify that additional separation process.

Compatibilizers also target specific resin types. No “one-size-fits-all” compatibilizer exists on the market today for any mix of materials. For compatibilizers to work consistently, the recycle feed stream itself has to be fairly consistent in resin composition. On the other hand, where there are varied recycle streams, the use of organometallic esters should be investigated more thoroughly and their efficacy established. Also, as with any new chemistry, food-contact approval for the compatibilizer use has to be obtained.

The SPI Recycling Committee and its members continue to explore the opportunities to unlock the value of mixed plastics streams with compatibilizers and have developed a new material additive guide to help. To download a free copy, click here.