Recycling of medical PETG packaging: incorporation into polyols suitable for polyurethane coating applications

The Plastics Industry Association (PLASTICS), Washington, and the Healthcare Plastics Recycling Council (HPRC), Marlborough, Massachusetts, approached Resinate Materials Group, Plymouth, Michigan, with an imperative industry need for recycling PETG (glycol modified polyethylene terephthalate) medical packaging, which currently has no wide-scale, viable mechanical recycling options and, consequently, often ends up in landfills rather than being recycled and reused.

Resinate Materials Group, with expertise in upcycling and converting recycled polyester thermoplastics and polycarbonate thermoplastics into useful polyester polyols for polyurethane applications, offered to demonstrate the viability of transforming this material stream into a useful polymer intermediate—a polyester polyol—via proprietary glycolysis technology. Furthermore, the polyester polyol made with this recycled PETG medical-packaging-based polyester polyol was used in a direct-to-metal coating application, and when evaluated in a side-by-side comparison to a recycled-PET-based polyester polyol, the former displayed very similar attributes.

GOING HIGHER

Each year, millions of tons of spent materials are deposited in landfills, and any further use they might offer is lost forever. At Resinate, we have developed proprietary technology that allows us to harvest these valuable materials and extend their life cycles by upcycling them into higher-value polyester polyols. PETG is a glycol modified PET and, like PET, it is a transparent, colorless (in the absence of pigments or dyes), thermoplastic copolymer of terephthalic acid (TPA), ethylene glycol (EG) and cylcohexane dimethanol (CHDM). Including the CHDM in the formulation with the other two components changes the chemical composition and makes the PETG polymer less hazy and less brittle, a characteristic that often is observed in PET when it is heated (http://blog.wheaton.com/pet-vs-petg-what-is-the-difference/).

Most of these PETG and PET feedstock materials that fall within a similar family of resins already have a significant energy history and environmental footprint that has been expended in their production and have, consequently, become global commodity materials because of their performance properties, according to G. Spilman, ACS Green Chemistry Institute, in Green Chem: The Nexus Blog, Nov. 22, 2016. PETG sheets have high stiffness, hardness and toughness, as well as good impact strength.

Today, PETG is commonly used for thermoforming applications. PETG affords packaging designers unique design freedoms, and the clarity of the material allows users to readily see the product. In medical applications, PETG is used for thermoformed trays, clamshell packaging, blister packaging, mounting cards, lids and folding cartons (www.polymerplastics.com/transparents_petg.shtml). Because of these desirable attributes, PETG is commonly a material of choice for packaging medical devices. However, this results in PETG packaging scrap commonly being generated and discarded in the clinical areas at hospitals, where very little recovery occurs (www.recyclinginternational.com/recycling-news/9586/plastic-and-rubber/united-states/new-high-us-plastics-recyclers).

It was the goal of the Healthcare Plastics Recycling Council (HPRC) and PLASTICS to test the recovery of prepatient plastics gathered from clinical areas from a network of hospitals in the Chicago area. (See www.recyclinginternational.com/recycling-news/9586/plastic-and-rubber/united-states/new-high-us-plastics-recyclers for more information.) The team explored mechanical, chemical and pyrolysis recovery solutions for the mixed forms of plastic packaging generated in these areas. Because of the lack of widely available markets for mechanically recycling PETG, the team contacted Resinate to explore chemical recycling options.

BUILDING ON EARLIER WORK

Incorporating recycled content in a polyester polyol formulation is not a new concept, according to V. Sinha, M.R. Patel and J.V. Patel in the Journal of Polymers and the Environment, Volume 18, Issue 1, March 1, 2010, pages 8 to 25. However, Resinate’s glycolysis technology employs a novel methodology for depolymerizing these plastic feedstocks into their oligomeric units, which may comprise a mixture ranging from dimeric to hexameric units, per A.M. Al-Sabagh, F.Z. Yehia, D.R.K. Harding, G. Eshaq and A.E. El Metwally in Green Chemistry 2016, Volume 18, Issue 14, page 3997, published April 12, 2016. Subsequently, by adding specific hydrophobes that may include mono- and multifunctional acids, the performance properties of the resulting polyester polyol can be dialed in to address very specific requirements that a customer might be seeking.

While PETG and PET are chemically similar as polyester-based polymers, these resins can be chemically processed via depolymerization as a glycolysis step. Processing temperatures are critical, and in mechanical recycling processes, plastics recyclers in particular have noticed the effect of these different processing temperatures when PET and PETG streams are mixed. In fact, in mechanical processing when these streams are combined, PETG and PET can act as contaminants for either stream of material. However, this is not true in chemical recycling. A demonstration project proved the Resinate process effectively can convert PET and PETG streams back into valuable chemical building blocks; and, unlike mechanical recycling, these polyester-based resins can be processed either as pure streams without PET and PETG cross contamination or as a mixture because both resins can be depolymerized simultaneously via its proprietary glycolysis process to produce a glycolyzed version of the resins. It is widely known that PETG is used as rigid thermoformed packaging for medical devices. The glycolysis methodology permits all forms of recyclables from clinical areas, as well as from other consumer packaging recyclables, such as bottles and food packaging, to be co-introduced into the reactor to produce a homogeneous glycolyzed version that can be either just PET or PETG or a mixture of the two.

It is the intent of this paper to eliminate one of these challenges by demonstrating that recycled PETG medical scrap has value as an ingredient in polyols used for specialty polyurethane applications.

EXPERIMENTAL

As part of the health care plastics recycling demonstration project, a mix of medical thermoformed PETG packaging materials was obtained from the main and outpatient surgery areas at Advocate Illinois Masonic Medical Center in Chicago. The collected materials were selected manually to avoid non-PETG materials, including avoiding paper labels, and were a visually uniform PETG material tinted with a blue pigment. HPRC delivered approximately 2 pounds of the materials to Resinate in an ungranulated state and was confirmed to be suitable as a viable raw material for glycolysis. However, because the glycolysis process requires a granule, flake or pelletized form, Resinate opted to use a postindustrial PETG medical packaging scrap available from a commercial recycler that was in flake form, measuring approximately 0.25 inches in diameter.

General synthesis of polyester polyol

The synthesis of the polyester polyol employing postindustrial medical PETG packaging material was conducted using Resinate’s proprietary glycolysis process technology. The experimental polyester polyol was essentially targeted for direct-to-metal coating applications. The polyester polyol includes 76 percent by weight green content. Resinate Materials Group defines green content as the sum of the recycle and biorenewable contents. It must be noted that a typical experimental polyester polyol made with recycled PET normally contains 39 percent recycled PET by weight; however, in the case with the recycled medical packaging material PETG when used as a replacement for the PET on a molar basis, the resulting weight percentage was 42 percent in the polyester polyol formulation. This additional amount of PETG accounted for the higher overall viscosity of the polyol, as it was found to be approximately 30 percent higher. After the polyol synthesis was complete, the polyols were diluted with 20 percent by weight butyl acetate as a means of reducing viscosity.

Synthesis of PETG Polyester Polyol (IMP1005-1.0)

A 2-L reactor was charged with glycerol, propylene glycol, neopentyl glycol, MTBO (catalyst) and 50 percent of the PETG thermoform. The experimental set up was completed by placing the reactor on a heating mantle under 0.2-cubic-feet-per-minute (CFM) nitrogen blanket with overhead stirrer and condenser. The stir speed was set to about 60 RPM, and the temperature was set to 200 degrees Celsius. The remainder of the PETG was added in four charges as the temperature was increased and the PETG started to melt and breakdown. The experiment was allowed to run for 2-and-a-half hours after all PETG was incorporated. Subsequently, the temperature was lowered to 100 degrees Celsius for the acid (succinic and isophthalic acids) additions. The condenser was swapped for a silver jacket column and short path condenser. The acids are added and the temperature was set to 150 degrees Celsius, while the RPM was set to 300. The nitrogen flow was increased to 0.4 CFM, and the temperature gradually was increased by 5 degrees Celsius when head temp was below 90 then by 10 degrees Celsius up to 205. Once the acid number was below 10 mg of potassium hydroxide per gram, the temperature was brought down to 120 degrees Celsius and n-butyl acetate was added. The product was allowed to mix thoroughly and poured out through a 225-micron paint filter.

Synthesis of PET polyester polyol (IMP1005-6.5)

The synthesis of the recycled PET containing polyol was conducted precisely as that described for the PETG polyol, except that recycled PET was substituted in lieu of PETG.

For synthesizing the PET containing polyester polyol, the entire PET (or 39 percent by weight of the total composition) was added at the outset of the experiment followed by the addition of the glycols. After complete incorporation of the PET that resulted in a homogeneous glycolyzed product, the diacids (succinic and isophthalic acids) were added to complete the formation of the final polyester polyol. For this product, too, the polyol was filtered through a 225-micron paint filter.

Direct to metal coating tests:

The following steps were taken to recover the polyols from the PETG sample and to create the surface coating product. The polyester polyols (about 15 grams) synthesized as described above, were added to a 250-milliliter beaker at room temperature. Hexamethylene isocyanurate trimer was then added to the beaker at a level corresponding to 1.10 hydroxyl/isocyanate ratio. The mixture was then diluted to 55 percent by weight with propylene glycol methyl ether acetate. Mechanical mixing was conducted using a tri-lobe agitation blade measuring 3 inches in diameter and was gradually increased until it reached 500 RPM and a homogeneous mixture resulted. Dibutyltin dilaurate (0.05 percent by weight) was then added to the reaction mixture. After approximately five minutes of reaction time, a bead of the liquid reacting mixture was applied to one side of each of five aluminum panels measuring 4 inches by 6 inches. The beads of solvent-borne polyurethane were then drawn down each panel into a wet film using a No. 50 R.D. Specialties drawdown bar to a wet film thickness of 4.5 millimeters. The panels were allowed to flash dry in a hood at ambient temperatures for at least one hour and then heated in an oven to 135 degrees Celsius for 30 minutes.

RESULTS AND DISCUSSION

The resulting rPETG polyol (IMP1005-1.0) properties were measured and compared with the properties of the original rPET polyol (IMP1000-6.5).  The results are shown in Table 1, below. Generally speaking, the polyol properties were quite similar, with the rPETG polyol being slightly more viscous, a likely conclusion, based on the higher weight percentage of the polymer used in the polyester polyol formulation. It was also noted that the rPETG polyol was visibly more opaque because of its relatively dark blue appearance (Figure 1, below). However, the Gardner color of this polyol was the same as the rPET polyol, both reported at a value of five. Note that this Gardner value is most suitable for nonshow applications such as primers or highly pigmented industrial coating applications.

Table 1: Comparison of rPETG and rPET Polyol Properties

Figure 1: From left, postindustrial medical PETG packaging scrap, polyester polyol made with medical PETG packaging scrap and rPET polyester polyol

The final dry film thickness was determined using a PosiTector 6000 (Defelsko Corp.) dry film thickness gage. Konig hardness was measured using ISO 1522 using a TQC Pendulum Hardness Tester (Model SPO500). Pencil scratch hardness was measured using ASTM D3363. Flexibility was measured using ASTM D522. Adhesion was measured using ASTM D3359. Stain testing was measured using ASTM D1308. MEK double-rub testing was conducted using ASTM D4752. Table 2 summarizes the results for testing of these polyurethane coatings.

Table 2: Comparison of rPETG and rPET Polyol Coating Properties

^*Windex^® is a product from SC Johnson and Family company

^§Betadine^® Betadine is a product from Perdue Products and is used as a topical solution

^¶Skydrol^TM  Skydrol is a product from Eastman and is used in aviation hydraulic fluids. 

*24-hour spot exposure with a saturated Whatman Grade 1 25-millimeter filter paper under a watch glass. Rated from 5-0 (5= no damage, 4= discoloration, 3= minimal blistering, 2= mild blistering, 1= severe blistering, 0= complete delamination.)

As can be seen in Table 2, the resulting rPETG polyol yielded a hard, flexible, tough, impact resistant coating that adhered well to aluminum. The rPET-based coating exhibited significantly better MEK double-rub performance, presumably because of its increased aromatic character. However, the rPETG based coating exhibited improved Skydrol resistance versus the rPET polyol. Additionally, it would be expected that the rPETG based polyol would provide improved weathering resistance because of a reduction in its aromatic content versus the rPET polyol.

CONCLUSION

The overall goal of this collaborative project between Resinate, HPRC and the Plastics Industry Association was essentially to present that medical PETG scrap materials, which have a significant energy history and environmental footprint that has already been paid for in their manufacture, can be upcycled and can be valuable assets in creating a sustainable economy. The performance of the medical PETG scrap when incorporated in a polyester polyol provided compelling reason to work for recycling this valuable raw material into specialty applications such as coatings. Further, it is believed that polyols containing rPETG can be used in polyols designed for polyurethane adhesives, sealants, elastomers, flexible foams, rigid foams, melamine based coatings and polyisocyanurate rigid foams.

This demonstration should provide an economic incentive and a practical reason to begin forming collaborations between end-of-use consumers of PETG packaging, recyclers and beginning-of-life PETG polyol producers to promote solutions that avoid landfill as an option for this valuable raw material.

Shakti Mukerjee, Mike Christy, Brian Reid and Rick Tabor are with Resinate Materials Group Inc., 46701 Commerce Center Drive, Suite C, Plymouth, MI 48170. Kim Holmes is with the Plastics Industry Association, 1425 K St. NW., Suite 500, Washington, DC 200052. Peylina Chu and Chris Rogers are with the Healthcare Plastics Recycling Council (c/o Antea Group), 400 Donald Lynch Blvd., Suite 104, Marlborough, MA 017523.