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Sustainable plastics from agricultural coproducts: Seven things to know

USDA ARS pallet molding at Tranpak
A California section of the USDA Agricultural Research Service (ARS) answers seven questions about its research in optimizing bioproducts to augment plastics sustainably.

Researchers with the United States Department of Agriculture’s Agricultural Research Service (ARS) Western Regional Research Center (Albany, CA) provide an update to their projects centering on optimizing bioproducts that PlasticsToday most recently reported on in September 2018, California’s going nuts over plastic recycling.

1. What are the exciting new developments with plastics and composites research?

USDA ARS: Several exciting developments on composite research are worth highlighting. One of the goals of the USDA ARS project is to scale-up torrefaction of California-derived biomass, such as almond or walnut shells, which are made into a charcoal-like material to displace carbon black. Commercial utilization of our torrefaction technology (torrefaction is defined in Question 3 below) involves preparing composites for various areas of industry; e.g., automotive parts, shipping containers, pallets, and building materials.

Previously, our studies provided insights into the impact of biomass feedstock characteristics and the level of torrefaction on the overall properties of the composites. The torrefaction concepts that came from this study were based on lab-scale results, which we documented in several scientific journal articles. Recently, we began a collaboration with Mega Machinery Inc. (Knoxville, TN) and FDS Manufacturing Co. (Pomona, CA) to scale-up from kilograms to kilotons. Our torrefaction/pelleting method is based on a patent-pending process developed at the USDA ARS Western Regional Research Center in Albany, CA. To fund this novel technology, the research team at the USDA helped write a grant with FDS Manufacturing Co. through the Department of Resources Recycling and Recovery (CalRecycle). In May 2019, CalRecycle awarded FDS Manufacturing Co. with $2.9M to help develop the team’s idea.

Another exciting collaboration revolved around using the resins created in the torrefaction/pelleting process to make plastic composites into extruded or molded industrial prototypes. The collaboration resulted in protocols for producing the prototypes.  Two prototypes in particular are being explored for their commercial viability: injection-molded shipping pallets and extruded slip sheets. Injection-molded pallets were prepared in partnership with plastic pallet provider TranPak (Fresno, CA).  The TranPak team utilized various torrefied biomass feedstocks at low concentrations as plastic fillers with post-consumer recycled polypropylene/polyethylene blends. 

Another collaborator, Repsco (Modesto, CA), blended torrefied biomass with recycled plastic to improve mechanical and thermal properties of slip sheets, laminated thin sheets of plastic that are placed under and between shipping pallets to prevent containers from slipping during shipping, as well as to protect the products.  Both projects demonstrated the use of torrefied biomass as a replacement for carbon black in these significant industrial products. Moreover, the project also showed-off the Federal Technology Transfer process whereby bench-top trial-and-error science moves from the laboratory into the commercial realm.

Injection molding of a polypropylene pallet containing 4% torrefied biomass at TranPak (Fresno, CA).

2. What was the focus of the CalRecycle grant?

USDA ARS: CalRecycle offered a grant through the Recycled Fiber, Plastic, and Glass Grant Program, a statewide initiative that was aimed at the reduction of greenhouse gas emissions. The program created financial incentives for industries, particularly located in disadvantaged communities, to invest in technologies to reduce pollution.

In March 2019, Kevin Stevenson, co-owner of FDS Manufacturing applied for a grant along with Mega Machinery through the CalRecycle program. In May 2019, FDS Manufacturing was awarded $2.9M to develop a process for diverting post-consumer, recycled polypropylene and polyethylene to create compounded masterbatch resin pellets. These resulting pellets will then be used by FDS Manufacturing and other industrial partners to produce commercial products such as pallets, agricultural storage bins, angle boards, and other multi-use products. A projected 4,005 tons of plastic is estimated to be diverted from landfills in this initial pilot study.

By adding torrefied walnut and almond shells, especially to recycled plastics (and less so to virgin plastics), we can increase the tensile and flexural modulus (i.e. stiffness) as well as the heat deformation temperature of the resulting plastics.  For applications like a pallet where a stiff plastic is desirable, using torrefied shells in recycled polypropylene allows the manufacturer to reduce costs because torrefied biomass is cheaper than recycled plastic. We also reduce plastic usage.  Plus, walnut and almond shells are a renewable bio-based filler.

3. What is torrefaction?

USDA ARS: Torrefaction is a thermal process, used most famously to “roast” coffee beans, in which biomass is heated between 200-300 °C in a low oxygen environment.  Three distinct final products are obtained: (1) a permanent gas, composed mostly of CO2 and CO; (2) a condensable liquid, high in water content, but often acidic; and (3) a carbonized solid. The solid has a higher hydrophobicity than the non-torrefied biomass, which means it overcomes issues with water uptake when using most agricultural fibers. Increasing the hydrophobicity of biomass filler through torrefaction improves the overall properties of the composites by improving adhesion of filler to plastic. Thus, torrefied biomass can be considered an attractive, eco-friendly, renewable and low-cost reinforcement filler for recycled, or “waste,” plastics.

Next: Biopolymer composites, what's next and more

4. What are concerns about using torrefied biomass filler?

USDA ARS: Hypothetically, some plastic recyclers may be concerned about collecting plastics with high levels of biomass filler within their processing stream because of concerns with moisture or unstable fibers diluting their recycling stream.  However, it must be conveyed that torrefied fillers are similar in structure to most typical carbon black fillers, just eco-friendlier. Plus, torrefied fillers are to be added at small concentrations, with typical concentrations of 2-5%, so they are equivalent in concentration to carbon black and other benign fillers.

A 48” x 48” polypropylene pallet manufactured with 4% torrefied biomass.

5. What can you say about biopolymer composites?

USDA ARS: Commercially available biopolymers are typically generated from agricultural feedstocks. Polylactic acid (PLA), in particular, is produced from corn and is becoming a widely used biopolymer due to its useful properties and biodegradability. It has been commercialized to compete with petroleum-based plastics because it can form clear, stiff products well and it can biodegrade in favorable composting conditions.

However, PLA does face restricted market penetration due to its low softening temperature and higher cost relative to petroleum-based plastics, such as polypropylene and polyethylene. The softening temperature is the temperature at which the plastic becomes pliable, and in its present unmodified, commercially-available form, PLA becomes too “pliable” at too low a temperature. This is where adding eco-friendly torrefied fillers may help, by reducing the cost and by broadening the application of PLA-based products. For example, increasing the softening temperature will allow the material to be used in an array of products, such as hot beverage containers, other single-use food items, mulch-films, coatings and even textiles. Even if the reduction in cost is minimal, it is plausible that consumers will still be willing to pay a slight premium for the environmental advantage that this biopolymer can offer.

6. Why focus on recycled materials?

USDA ARS: Because China no longer accepts plastic waste from the U.S. along with the fact that some recyclers have even resorted to landfilling some waste plastics, it is important to try to find ways to recycle these materials. Fortunately, polypropylene, in addition to other plastics such as polyethylene terephthalate (PET), polystyrene (PS), and polyethylene (PE), can be recycled. Reclamation of these plastics allows for their conversion into a variety of new materials.  However, reclamation often produces materials with reduced mechanical and thermal properties. This is loosely termed “down cycling” the plastic rather than “recycling” and it affects the end-use value of the reclaimed product.

To improve or broaden the range of properties of the reclaimed materials, certain additives are melt-blended via extrusion into recycled plastics. These additives typically include glass fibers, calcium carbonate, and elastomers (e.g., natural rubber).

For example, glass fiber-filled composites have high stiffness, good weight-to-strength ratio, and high impact strength which make them ideal for interior as well as exterior automotive parts. Talc powder, a commonly used industrial filler, can also increase stiffness and mechanical strength of reclaimed plastics.  It can also displace the cost of adding virgin material to recycled plastics. Other additives such as carbon black, talc, and titanium dioxide are also used; however, they are typically added as colorants and offer very little improvement to the mechanical properties of recycled plastics.

This project demonstrated that compounding recycled (“waste”)  plastic with torrefied biomass produces a composite material with equal or improved properties relative to virgin polymer. The biomass is derived from sustainable, natural resources and can reduce greenhouse gas emissions considerably.  Moreover, the handling of biomass yields less health and safety hazards and produces much less wear on processing equipment, unlike, for example, glass fiber-filled recycled plastic composites.

7. What’s next?

USDA ARS: We’re excited to find new companies to work with to develop and optimize composite blends for their commercial plastic products.  Currently, we are in talks with an automotive manufacturer to create plastic composites using our torrefied biomass. We also collaborating with another company that manufactures composite outdoor decking. Its eco-friendly products are very much in line with our own eco-friendly torrefied biomass fillers. As we continue to move forward with scaling up the technology, we want to find early-adopters willing to incorporate the fillers into their products.

See also Nutty research: Almond shells sustainably strengthen plastics, published August 2018.

Researchers left to right:

William Orts, Ph.D., Research Leader for Bioproducts at the USDA-ARS Western Regional Research Center, Albany CA, leads a team of researchers that optimizes strategies for adding value to agricultural coproducts, especially from biomass sources found in the Western U.S. Dr. Orts takes great pride in helping companies realize first-of-its-kind commercial ventures, with a special emphasis on creating novel plastics, composites, nanofibers and biocomposites.

Zach McCaffrey, Ph.D., Chemical Research Engineer, Bioproducts, is passionate about reducing waste and increasing waste utilization. Current projects include using bio‐additives to improve thermal and mechanical properties of polymer composites, lab‐scale torrefaction reactor design and scale‐up, and almond hull sugar and phenolic extraction.

Lennard Torres, Chemist, Bioproducts, specializes in the synthesis, modification and analyses of biopolymers to improve the properties of commercially available products. He is also a co-inventor of four patents in collaboration with the U.S. Department of Agriculture.

Ms. Delilah Wood, Botanist, Bioproducts, is the Characterization Facility Manager and Lead for nut co‐products research and development team. Her research includes bioplastics, guayule rubber, cereal science, food science, nutrition, extrusion technology, entomology, and food safety using microscopes and characterization equipment.

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