The pressure for landfill diversion strategies have increased with consumer awareness of traditional end-of-life practices and environmental accumulation. Brand owners, retailers, and regulatory bodies have voiced the need for increased post-consumer recycled polymers in consumer and industrial goods. It is therefore critical to understand the influence of post-consumer recycled polymer (PCR) content on properties and consumer safety as converters blend PCR with virgin resin to increase sustainability. Regulatory bodies have instituted minimum post-consumer recycled (PCR) content laws to increase the long-term sustainability efforts of polymer use in commodity goods (e.g., SB 270 in California); thus, the ability to empirically quantify the post-consumer recycled content is vital for compliance.
Furthermore, such methods enable brand owners and retailors to qualify their suppliers based upon empirically derived data instead of potentially mislead specification sheets. There are two main modes currently thought to provide empirical quantification of post-consumer recycled polymer content for certification purposes:
1) In-process monitoring of products during conversion
2) Testing of unknown “off the shelf” products.
This white paper describes our efforts to enable the empirical testing of post-consumer recycled polymers to enable the industry to comply with regulations and offer traceability of more sustainable solutions. We have demonstrated trends between the measured properties and PCR content which are identified potential certification markers for empirical quantification of PCR content and single-measurement quality control metrics.[Fig.1-5] To date, our work has been focused on polyethylene terephthalate and polyethylene polymers, however, this work will expand into other thermoplastic polymers, such as, polypropylene and polystyrene.
The need for an enforceable post-consumer recycled polymer standard
Recyclable consumer-generated and post-industrial solid waste is mostly sent to municipal recovery facilities with all other materials sent to landfills in the US. Consumer and environmental groups recently have become much more vocal with respect to the negative impact so plastics on the environment and have generated a strong push for industries to become more environmentally responsible. Most plastics (synthetic polymers) degrades slowly in the environment due to the lack of highly labile bonds in the molecular structure; thus, increasing production of consumer goods with virgin feedstocks increases the potential of environmental accumulation such as the Great Pacific Garbage Patch and roadside litter. To decrease the detrimental environmental impacts associated with plastics use in everyday life at home, schools, offices, arenas, and factories, methods for managing large quantities of contaminated plastics (mixtures of plastics, metals, food wastes, etc.) are required . Although the collection and sorting of large amounts of mixed municipal waste still poses a challenge for large-scale recycling in the US, a significant amount of plastic is recycled and reformed into consumer goods annually . Designing large-scale recycled products will require that the raw materials, fabrication, manufacturing processes, finished products, and plastic components reclamation needs to be considered before they enter the market.
In addition to landfill diversion, recycling has been estimated to provide enormous energy saving potential over the production of virgin resin which provides additional environmental benefits other than waste diversion ; for example, it has been estimated that recycling HDPE consumes ~24 % less energy than the production of virgin HDPE resin. In 2014, the reclamation of thermoformed PET containers was over 100 million pounds which was 70% more than in 2013 . Utilizing post-consumer recycled (PCR) content in food packaging has the potential to increase the added value of packages as a sustainable alternative to those manufactured from virgin resins. In some cases, the value of sustainability may generate a preferred vending status, as with the Walmart Packaging Scorecard . Although recycling is a household term, the Food and Drug Administration has three distinct classifications of plastic recycling: primary recycling is pre-consumer, post-industrial scrap that is internally ground and is routed directly back into the processing stream; secondary recycling is post-consumer which requires grinding and washing prior to reprocessing; and tertiary recycling which is a chemical process that deconstructs the polymer back into the precursors (monomers). Although primary recycling technically diverts waste from landfills, it has been commonplace that manufacturers utilize post-industrial scrap as feedstocks for products made from virgin resin. Most post-consumer reprocessing in the United States involves separating, grinding, and washing, followed by melt processing (e.g., extrusion, injection molding) into a new product.
Over the last 13 years, our research group has generated a vast amount of data regarding the influence of post-consumer content on the physical properties and food contact safety of polyethylene terephthalate (PET) and polyethylene polymers. PET is widely used for packing liquids (carbonated beverages, juices, and oils) and fresh-cut produce due to its excellent barrier, strong mechanical, and high transparency properties [10-13]. High-density polyethylene (HDPE) is widely used in agriculture, packaging, and grocery bag applications. HDPE consumed ~38.4 % of the overall virgin thermoplastic bottle market in 2015 second only to polyethylene terephthalate . In 2015, approximately 34 % of the 1.1 billion pounds of post-consumer resin collected from bottles were recycled, and a vast majority of agricultural waste was either burned or landfilled [14, 15]. As the overall production of both virgin feedstocks of both polymers greatly increases the potential for environmental accumulation, there is a strong drive to collect these polymers from the waste stream for recycling. The need for certification markers to quantify post-consumer content in “off the shelf” products manufactured from unknown manufacturers is critical for sustainability certification and compliance with California law (SB 270/ Proposition 67) [29, 30].
Thermo-mechanical processing of polymers results in main chain degradation of the polymer, resulting in a multitude of degradation by-products. The relative concentration of each by-product depends on the environment, polymer type, and residual metal content (either contamination or residual catalyst).[16, 17] Such degradation is accelerated in post-consumer polymers due to the presence of additional photo-induced degradation sites and the previous melt processing step and thermo-oxidized sites. For polyethylene terephthalate polymers, thermo-oxidation is known to occur at the diethylene glycol constituent forming a peroxide compound (Scheme 1, left). The resulting hydroxyl radical reacts with the benzene ring of the terephthalic acid constituent to produce quinone/hydroquinone derivatives.[18-21] An alternative degradation mechanism can produce alkene and aldehyde functional groups via β-scission of the carbon-oxygen bond adjacent to a carbonyl group.[19, 20].
Scheme 1. Left: Reaction pathway of diethylene glycol constituent under thermo-oxidative conditions of poly(ethylene terephthalate) [2, 19]. Right: Schematic mechanism of light induced PE degradation to form oxygenated moieties .
The mechanism of polyethylene degradation is generally accepted to proceed through alkyl and alkoxy radical intermediates from cleavage of the polyethylene backbone . As the carbon hydrogen bond is too strong to directly be cleaved by ultraviolet and visible light, the degradation process is photo-initiated from residual initiator compounds as well as peroxides that formed during the melt-processing conversion into the final article. The relative amount of chain scission events to crosslinking events in an inert atmosphere is influenced by the presence of various catalysts and heavy metal contaminants . In an oxygenated atmosphere (air), however, alkyl radicals react with oxygen to form a variety of oxygenated species (Scheme 1 right; ketones, aldehydes, esters, carboxylic acids, hydroxyls) [22, 23]. Generally, the two main oxidation products of polyethylene from either photo or thermal oxidation events are vinyl and carbonyl compounds [25, 26]. The carbonyl characteristic bands of infrared spectra have further been deconvoluted into the various carbonyl species which the distribution has been shown to be influenced by the exposure conditions . However, many studies have evaluated the carbonyl content in more general terms through calculation of the carbonyl index [24, 25, 27, 28].
A step towards an empirical standard for quantifying the post-consumer content of all thermoplastic polymers
As the drive to divert consumer goods comprised of polymeric materials from landfills increases due to increased consumer awareness, the pressure for brand owners and manufacturers to utilize post-consumer recycled polymer materials in packaging and products increases. In some examples, regulatory bodies mandate that specific products, such as carry out bags (CA SB 270), must contain a minimum post-consumer content, thus, it is critical that an empirical protocol be developed to establish a method for converters and brand owners to submit products for compliance analysis. Our previous work in post-consumer/virgin polyethylene terephthalate (PET) polymer blends demonstrated the ability to utilize measurable chemical and physical changes (e.g., infrared and ultraviolet-visible absorption, melting properties, fluorescence properties) to calculate the post-consumer content in the final part [2, 4, 5]. This observation prompted the investigation of utilizing similar measurable properties to determine the post-consumer content of other polymer blends, such as polyethylene of the various densities (HDPE, LDPE, LLDPE).
To be able to establish an empirical protocol to quantify the post-consumer content of “off the shelf” products, it is critical to generate multiple series of post-consumer/virgin polymer blends of specific polymer types (e.g., PET, HDPE, LDPE, PP). In the vast majority of our work, we melt process our own PCR/virgin polymer blends via melt extrusion; in the example above, the post-consumer resin was received as simply flaked and washed granules of high-density polyethylene, although, reprocessed and pelletized PCR has also been received. The PCR polymer is blended by hand with virgin resin to specific blend concentrations then converted in film or sheet. To compare the properties that result from processing on a pilot scale compared to the industrial scale extruders, blends of virgin and post-consumer polymers are obtained from a commercial supplier and the properties measured.
A variety of statistically analyses have been utilized to produce multivariable equations for the different polymers, including: stepwise multivariable regressions, LOGIT, and PROBIT. Inputs into the various models have been generally spectroscopic measurements, i.e., colorimetric data, ultraviolet visible absorbance, Raman intensities, and infrared absorbance which traditionally provides rapid analyses. Other physical properties, such as melting properties and mechanical properties have also been demonstrated to vary as a function of PCR content but require benchtop type analyses. Indeed, complementary techniques (spectroscopic, physical, and thermal) would contribute to increased robustness of the measurements due to separate, yet interdependent, changes in properties of the blended PCR polymer, relying on spectroscopic data increases the potential to use multivariable equations in-process to quantify and monitor post-consumer content during conversion of real parts.
A multitude of measurable parameters have been identified in our work that have potential to be utilized as certification markers of post-consumer content for empirical quantification [1, 2, 4]. Our ongoing research advances our previous models for increased robustness by increasing the number of data points and long-term challenging of the existing models. As more data are collected and challenged, each model is statistically re-evaluated and challenged to increase the accuracy and precision. The empirical determination of post-consumer content from measurable parameters using the approach described here has potential for identifying unknown compositions from “off the shelf” products and enables certification companies and regulatory bodies to empirically test specific articles and rely less on formulation and purchasing logs.
Post-consumer polymers are well known to reduce the overall energy consumption of production and minimize the detrimental impact of polymer consumer goods on the environment. With state regulatory bodies and brand owners increasing the pressure to utilized post-consumer polymers in everyday applications, the need for an empirical method to demonstrate compliance is critical. Our research has clearly demonstrated the ability to test “off the shelf” products to empirically determine the post-consumer content of a specific specimen for both polyethylene terephthalate and polyethylene polymers. Using empirically derived, and complementary, property analyses of PCR/virgin polymer blends, certification of blended materials are possible. The benefit of using complementary techniques has two independent qualities, 1) the spectroscopy technique can be employed throughout the manufacturing with high throughput quantitative inspections, and, 2) thermal and physical property analysis reassures the quantitative aspect of the spectroscopic measurements based on separate, but interdependent, properties of the blended PCR polymer. Furthermore, these techniques have the potential to monitor and provide instantaneous feedback to the manufacturer to control material composition in real-time to achieve the desired properties of the final product during processing. The methods to quantify the post-consumer content are explicitly listed in U.S. patents 9638683 and 8063374 which are both owned by the Cal Poly Corporation.
Ongoing investigations into the ability to create more robust certification models for polyethylene terephthalate, polyethylene, and other thermoplastic polymers of differing densities from differing virgin and mixed-stream post-consumer feedstocks will enable the widespread adoption of empirical quantification of post-consumer content for certification and compliance purposes. Understanding the influence of polymeric contaminants in mixed-stream feedstocks (e.g., polypropylene, polyvinyl chloride, poly(lactic acid) and additives (e.g., pigments, thermal stabilizers, processing aids, light stabilizers) is critical and the subject of current research throughout the group. Although the efforts to date have focused on film and sheet applications (e.g., trash bags, agricultural films, carryout bags, clamshell packaging), this technology has demonstrated potential for quantification of post-consumer content in other industries, such as automotive and civil engineering where sustainable options and long-term performance are required.
The research described above was funded and supported by IdeoPak, LLC, the Polymer and Food Protection Consortium at Iowa State University, and California State University, San Luis Obispo which granted permission to use the intellectual property patented and owned by the university.
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Dr. Keith Vorst serves as the Director for the Polymer and Food Protection Consortium in the Department of Food Science and Human Nutrition at Iowa State University in Ames, Iowa. For more information please visit his Bio Page.
Dr. Greg Curtzwiler received a B.S. in Biochemistry and M.S. in Polymers and Coatings Science from Cal Poly State University, San Luis Obispo, CA. He received a Doctorate in Polymer Science and Engineering from The University of Southern Mississippi School of Polymers and High-Performance Materials.
For more information please visit his Bio Page.