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Gary Spilman, Ph.D., Principal Scientist, Resinate Materials Group, Inc.
In 2013, the U.S. produced 9.4 billion pounds of plastic bottles, yet less than 31 percent of them were recycled. That means 6.5 billion pounds of plastic bottles are destined for landfills or incinerators. Many of these bottles are made from poly(ethylene terephthalate) (PET) and poly(bisphenol-A carbonate) (PBAC). These are materials that already have a significant energy history and environmental footprint paid to that point, and have become global commodity materials because of their performance properties.
PET offers excellent chemical resistance (semi-crystalline), low water uptake, high tensile strength and modulus, and excellent flexural strength and modulus [1]. PBAC offers extremely high impact strength, good chemical resistance, along with very good toughness [2]. These properties combine to make PET and PBAC excellent engineering materials. In addition, they are also a gateway to well-balanced, superior performance attributes (and some surprising advantages) as lower molecular weight polyols for demanding coating, adhesive, sealant, elastomer, plasticizer and specialty foam applications.
Although the use of recycled content is not a new concept, Resinate Materials Group (RMG) uses novel technology that allows one to build in unique and customizable performance. Through a proprietary process, PET and PBAC are broken down through glycolysis to an oligomeric state. Subsequently, these oligomers can be reassembled using unique combinations of mono- or multifunctional acids and other hydroxyl reactive building blocks. The result is a variety of specialty polyols that can be customized to specific performance needs. These unique materials are delivered from discarded engineering plastics at a cost comparable to traditional specialty polyols.
RMG’s approach to molecular design is best described as a hierarchy of components that are selected for integration with the oligomeric intermediates from glycolysis. This hierarchy dictates the foundation for raw material selection. The initial (and usually primary) ingredient choice is, of course, the digested recycled materials described. Second-priority sourcing is based on biorenewable components, a list that is constantly expanding. This includes such ingredients as levulinic acid derivatives, lignin, vegetable oils, dimer and trimer fatty acids, and sugars. Lastly, incorporation of appropriate intermediates from virgin petroleum sources may be necessary to produce the highest performance from polyols that can contain close to 100% “green” content (recycled and renewable).
RMG’s polyesters and hybrid systems utilize their high levels of recycled content from solid state plastics that have been processed into small flakes, chunks, pellets, or fibers, from sources such as consumer packaging, carpet and automotive components. As mentioned previously, the RMG process is unlike some tertiary (chemical) recycling processes using hydrolysis, methanolysis, or ethylene glycol glycolysis [3], [4], since we do not depolymerize to the monomeric state or target high yields of bis(2-hydroxyethyl) terephthalate [5], [6] (BHET, mol wt 252). This would amount to “over-processing” beyond the point of optimal usefulness for coatings. For example, one could synthesize virgin aromatic polyesters containing varying amounts of terephthalic acid and ethylene glycol. However, these materials would be architecturally very different despite being compositionally similar to some RMG polyesters based on recycled PET content. This is because the initial digestion phase produces building blocks, which are far from monomeric. The glycolysis process is controlled by several factors, including overall time, temperature, catalyst type and level, and glycol-to-PET ratio. The figures (Figs. 1 and 2) are visual distributions of the resulting molecular weights from glycolysis of PBAC and PET respectively. For each system, the percentages of high oligomeric species (mol wt >1000) are noted based on detector area analysis.
Short “blockiness” in the polyester backbone can result from these various oligomeric constituents, and will, in turn, vary with the polydispersity of their prior glycolysis. Building up from the low-moderate molecular weight oligomers to high-performance macromolecular specialty polymers is the synthetic goal at RMG. In addition to PET and PBAC, we have also performed similar process steps using a variety of other recycled thermoplastics, including glycol-modified PET (PETG) and poly(trimethylene terephthalate) (PTT).
The most important design criteria for a given polyol product is its performance, since market value and success for this technology is primarily dependent on customer acceptance, validation, and use. RMG has coupled this performance with their goal to re-deploy and elevate the “used” molecules to their highest value. The recycled materials will be truly “upcycled” in this way, ultimately becoming transformed for a higher value purpose as a protective coating, adhesive or other specialty application. These new assigned applications may function for decades, as opposed to months as a commodity packaging material.
Beyond diverting consumer waste streams, there are additional streams available where alternative sources for valuable key raw materials may be found, such as those classified as “post-industrial recycle.” In a similar fashion, these waste materials can be recovered and incorporated into a variety of products. Surprisingly, some can occasionally be converted back to useful materials for their starting industry, and re-used within their original manufacturing processes and products. This results in a truly circular business model, where industrial waste streams can subsequently become a key input for RMG, to then convert into a key input for the initial industrial process - ultimately closing the loop and contributing to a circular economy approach to chemicals.
For more information about Resinate® polyols and licensable technology, visit www.resinateinc.com or call +1 800-891-2955.
Figure 1. Typical size exclusion chromatogram for digested recycled PBAC (70% > 1000 Mn)
Figure 2. Typical size exclusion chromatogram for digested recycled PET (50% > 1000 Mn)
[1] http://www.plastic-products.com/part12.htm
[2] http://www.ptsllc.com/intro/polycarb_intro.aspx
[3] Bartolome, L., Imran, M., Cho, B.G., Al-Masry, W.A., Kim, D.H., Recent Developments in the Chemical Recycling of PET, Material Recycling – Trends and Perspectives, Achilias, D., Ed., 2012, InTech, DOI: 10.5772/33800.
[4] Paszun, D., Spychaj, T., Ind. Eng. Chem. Res. 1997, 36, 1373.
[5] Al-Sabagh, A.M., Yehia, F.Z., Harding, D.R.K., Eshaq, Gh., El Metwally, A.E. Green Chem. 2016, 18 (14), 3997.
[6] Khoonkari, M., Haghighi, A.H., Sefidbakht, Y., Shekoohi, K., Ghaderian, A., Intl. J. Polym. Sci. 2015, Article ID 124524.
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