Contributed by Dr. Gary Spilman, Principal Scientist, Resinate Materials Group®.
Environmental, health and safety concerns continue to drive rapid growth for environmentally friendly, low VOC coatings. This growth, further compounded by increased social awareness of mega trends such as depleted finite resources, the growing world population, and constrained food resource, has companies seeking highly sustainable feedstock solutions. Although bio-based materials have provided feedstock options which are more sustainable than fossil petroleum alternatives, use of recycled content has remained relatively unexplored. With this in mind, Resinate Materials Group® has developed proprietary technology, which allows us to create multi-functional coatings using recycled raw material streams, including recycled poly(ethylene terephthalate) (PET). By harvesting materials otherwise destined for landfills we are able to extend the lifecycle of valuable, finite resources. Furthermore, studies have shown recycled PET feedstock to have more favorable life cycle assessment scores than comparable fossil petroleum-based or bio-based PET feedstocks. By harnessing the inherent properties of recycled PET, Resinate® has been able to impart a unique balance of properties into a variety of functional polyols and coatings including excellent hardness, good flexibility, and good chemical and stain resistance, all while developing a highly sustainable feedstock option.
With the U.S. production of plastic bottles at an amazing 9.4 billion pounds in 2013, and the total plastic bottle recycling collection rate at only 30.9%, there is a wide gap in unclaimed, uncollected, and discarded plastic bottlesi. The math is staggering when one considers that the remaining 69.1% amounts to 6.5 billion pounds. Where is the unclaimed material going? Landfills and incinerators take in much of the excess.
We have markets and supply chains for recycled polyethylene terephthalate (PET) bottles, but this option is operating inadequately and competing with virgin material for demand. There is a need for other options to become available for used PET material to live a second life as high-performance polyester, which its pedigree supports. One such option for these materials is their incorporation into high-performance protective coatings for wood and metal. Through careful synthetic breakdown and reassembly their lives as durable, functional, tough, attractive coatings can be realized. This new use for materials which were previously harvested and discarded, will help create increased demand and reduce the overall amount that finds its way into a landfill or incinerator.
PET, which has been processed into bottles, already has a significant energy history and environmental footprint paid to that point. With that impact and footprint already within the material, this recycled PET can reduce the need for creating additional impact, which would occur with the harvesting and processing of additional virgin petroleum-based feedstocks. We discuss here the means to reclaim used PET as a raw material in high performance coatings and the surprising results that accompany high incorporation of previously “spent” materials.
Starting with the recycled PET (rPET) stream, there are several parameters to consider when converting the bulk material into a useful form for coatings. The functionality and equivalent weight required for most coating polyols is not inherent in the rPET as supplied, so hydroxyl end groups need to be generated. This provides a handle for both secondary processing polyurethane dispersions (PUDs) and for final curing for thermoset coatings (melamine, isocyanurate, etc.). Since PET is inherently semi-crystalline, it is also necessary to determine whether to preserve or eliminate this property. According to Schiraldi, et.al., ii“modifying substances” can be used to affect the properties and degree of crystallization, tune tensile and modulus properties, adjust the Tg and Tm, and modify barrier properties. Carefully selected comonomers can accomplish this while simultaneously contributing to other performance attributes. From the aromatic side, isophthalic acid (PIA) has become the most widely accepted modifier for packaging applications due to its relatively minor effect on the Tg, reduction in the crystallization rate but not in the ultimate level of crystallinity (at < 5 mol%), and improved barrier properties. iiiAdditionally, hydroquinone and 4,4’-bisphenol are known to accelerate crystallization rates over neat PET.
The introduction of long chain diols can impart desirable characteristics such as flexibility. Polyols such as hexanediol, butanediol, and dodecanediol are good examples. Polyethers such as polyethylene glycol (PEG) or poly(tetramethylene ether) glycol (PTMEG) are also good diol modifiers for flexibility. This increase in flexibility may come along with substantial changes in Tm, Tg, and crystallinity. As is true with many properties, opposing ends of the property spectrum must be balanced to maintain good overall performance in coating applications. Additionally, in starting from mixed recycle streams of PET, there may be some unwanted color associated with prior use in packaging, and this may need to be eliminated for some coating applications.
Clearcoat layers designed as final topcoats for wood and metal substrates are normally colorless, and decolorizing rPET streams has become necessary for consistency. A novel process has been established for reducing or eliminating color associated with recycle-grade bottle flakes, but it will not be discussed here.
A final note on the design of polyols relates to natural and bio-based modifications. These ingredients are also of high interest and may include many different acids or anhydrides such as adipic and succinic, and diols such as propanediol, ethylene glycol, and others. Multifunctional intermediates such as pentaerythritol (Voxtar™)iv are now being made through a renewable and sustainable bio-based process, and can provide needed hydroxyl functionality for coating applications. Of course, most all fatty acids are naturally-derived and can provide some level of hydrophobicity in the polyol when needed. Polyether polyols have also found some level of “green” with alkoxylated hydroxyl-functional natural oilsv and epoxidized methyl oleate polyether polyols. viResinate’s® corporate philosophy with respect to green chemistry is to use recycle content raw materials first and biorenewable content second. If performance requirements set by our customers can't be met or exceeded with these first two options, only then do we use petroleum content raw materials or ingredients. This approach leads to the highest “green” content possible in the final polyol.
With the motivation to take advantage of all performance properties of PET, our company has developed new polyols from recycled PET that have demonstrated superior performance for coating applications. Resinate® polyols, when tested against conventional specialty polyols, clearly show desirable performance in the most popular wood and metal coating test categories. Hardness, flexibility, toughness, strength, and chemical resistance are all benefits Resinate® has taken from materials that have been previously spent for their designed purpose. The polyester material is waiting to be re-engineered for a new life as a coating. Resinate® is acquiring the performance data and design feedback from its process and composition variables to meet and exceed the needs in the coating resin sector with a high metric for sustainability.
Each year, millions of tons of used petroleum and other products are deposited in landfills, and whatever further use they might offer is lost. Resinate® recaptures those products as raw materials, turning waste streams into high-performance polyol solutions. We develop innovative ways to divert landfill waste, extend the life of finite resources and upcycle used molecules into high-performance polyester polyols — the backbone of coatings, adhesives, sealants, elastomers, foams and lubricants. For more information, visit www.resinateinc.com.
i2013 U.S. National Post-Consumer Plastics Bottle Recycling Report; The Association of Postconsumer Plastic Recyclers; American Chemistry Council, 2014.
iiSchiraldi, D., Scheirs, J., & Long, T. (2003). New Poly(ethylene terephthalate) Copolymers. In Modern Polyesters (pp. 245-265). John Wiley and Sons.
ivSvensson, C. (2011, March 29). Discover Voxtar™, world’s first renewable pentaerythritol platform, and more Perstorp sustainable solutions. Retrieved December 1, 2014.
vJack Reese., Stanley Hager., Micah Moore. (2012). US20140024733 A1. Pittsburgh, PA: United States.
viLligadas, G., Ropnda, J.C., Galia, M., Biermann, U., Metzger, J. O., J. Polym. Sci. Part A: Polym. Chem., 2006, 44(1), 634-645.
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