High Performance Coating Materials from Recycled Sources

Environmental, health, and safety concerns continue to drive awareness and interest toward improving the sustainability of materials used in the coatings industry.1 This is a subset of the larger movement toward becoming better stewards of global resources across all industries and working toward building a circular economy. To that end, this article discusses a nonpetroleum, non-bio resource for producing high performance industrial coatings resins. Alongside, and in harmony with current biobased efforts, Resinate has introduced commercial polyols that are designed to contain high levels of recycled materials. Previous presentations and publications2 introduced some of the descendants from recycled PET. Resinate now introduces multi-functional materials as base resins or blend additives, which are based on polycarbonate streams. Ultimately, these novel polyols provide a green chemistry alternative to heavy metal corrosion pigments, are nontoxic, can be added during the letdown stage of coating production, and can enhance other coating performance properties. This research compares performance in two separate formulated coating systems, and demonstrates the utility of these materials for metal protective coatings.

Efforts have been underway to replace conventional petroleum-based feedstocks with newer biobased versions of the same materials. Although these biobased materials have provided feedstock options that are more sustainable than fossil petroleum alternatives, use of recycled content has remained relatively underutilized for high performance coating applications. To that end, we have tapped a nonpetroleum, non-bio resource for producing high performance industrial coatings resins. Along with current biobased efforts, commercial polyols have been introduced that were designed to contain high levels of recycled materials. These recycled materials that are normally destined for the landfill or incinerator can alternatively be upcycled and reprocessed into protective coatings that otherwise might incorporate virgin petroleum products.

The ability to reprocess thermoplastic materials mechanically and chemically opens many doors for reducing waste volume. The digestion of high relative molecular weight (Mr )3 polyesters is not a new concept.4 There have been many documented efforts, both past and present, in the reuse and repurpose of both industrial and consumer waste polyesters.5 The ability to melt process bulk thermoplastics back into useful forms such as pellet or flake for incorporation into new finished goods is a very convenient output for the collection, accumulation, and separation of such materials. According to the Association of Plastics Recyclers (APR), while the collection of HDPE bottles in 2014 rose to a rate of 31%, totaling nearly 1.1 billion pounds in the United States,6 the total weight of postconsumer polyethylene terephthalate (PET) bottles collected for recycling in the United States in 2014 was 1.8 billion pounds.7 The most widely recycled plastic is PET,8 a major component of many common consumer products including Water and soda bottles, textiles, carpeting, auto interiors, and many packaging products.

Other materials, such as poly(bisphenol A carbonate), or PBAC, polyethylene, Polyvinyl chloride, polypropylene, and polystyrene, can all be reprocessed through mechanical means such as crushing, shredding, chopping, etc., and can also be ultimately reprocessed in the melt into similar molded objects from which they came. Although they all have this in common, only the condensation materials, PET and PBAC, have a direct route to chemical breakdown due to the ester or carbonate linkage that holds them together, allowing them to be upcycled into higher-value, longer-life applications. Other materials are much less suited to this type of breakdown due to their chemical constitution, having an all-carbon backbone with no alternative functionality for targeting chemical attack. This makes them less prone to environmental breakdown as well. To utilize plastics recycle streams and leverage their chemical heritage, this chemical breakdown access is needed as a way to redesign the bulk properties of the materials. This is necessary to bring them into a useful range appropriate for the final application. For coatings, this mainly targets the glass transition temperature (Tg ) and the viscosity of the material. The chemical breakdown method is a gateway to designing lower Mr  materials from their higher Mr  predecessors, while providing building blocks for further optimization.

As previously mentioned, polyester has been taken through these modification steps; in many cases, all the way to its monomeric state, providing ethylene glycol and Terephthalic Acid.9 However, PBAC has had very little attention in this respect, especially for coatings applications. One of the major limitations with PBAC as a feedstock is its tendency to liberate bisphenol A (BPA) at high process temperatures, if proper catalyst is used, and especially when water is available. We have looked into this process, and have designed products that have excellent properties for coatings. These are produced through the degradation of high Mr recycled PBAC and result in useful polyols for metal protective films. They demonstrate good corrosion resistance and are helpful in many film properties that are desired for metal protection. The introduction of intermediates based on recycled PBAC has provided new alternatives to heavy metal anticorrosion pigments used in films for environmental barrier protection.