Foam The demonstration
Foam The demonstration of these favorable properties and biobased origins still lacks an explanation for circularity. A solution for a polymer’s end-of-life that enables the recycled material to re-enter the values stream is the holy grail of a circular lifecycle. To recover valuable recyclates from the NIPU foam, chemical recycling was used by targeting the molecular design incorporating organic carbonates. The initial reaction of lignin with organic carbonates creates etherified and carboxylated chains extending from the lignin structure that can be used as molecular handles to unravel the polymeric structure. In addition, the urethane bond has been shown to be capable of depolymerization in alkaline conditions [3]. Alkaline hydrolysis proved to be an efficient method of depolymerization allowing for the recovery of lignin and diamine through precipitation and solvent extraction. Most importantly, the researchers have been able to show that the original hydroxyl content of lignin is partially restored during chemical recycling, an important step in confirming that the recycled precursors have similar properties to virgin materials. Indeed, the biggest hurdles in using chemical recycling are the side reactions associated with lignin’s own reactivity described earlier. To mitigate against the tendency of lignin to condensate into insoluble material of lower value, additives can be used during chemical recycling that protect the lignin structure and render it more valuable in its recycled form. Using this unique recycling process, it was possible to resynthesize the NIPU with 100% recycled content. The study aimed at synthesizing 100 % biobased, non-toxic and recyclable PUs has taught the scientists many lessons. The first is an old one: when given lemons, make lemonade! Instead of seeing lignin’s own reactivity as a roadblock, why not harness its reactive nature to produce reactive precursors? Finding a path to lignin functionalization was the key to producing PUs that can compete with commercial materials. Secondly, when designing a circular lifecycle look towards nature’s own path for degradation. The rich amount of carbon-oxygen bonds inserted during lignin functionalization mimic the natural decomposition pathways of typical biomass. While an enzymatic route was not taken in this approach (typical of composting mechanisms) it was still possible to use a benign hydrolysis technique to revert waste foams back to usable precursors. These innovative techniques do not have to be reserved only for PUs. The goal and philosophy of the Clemson team is to incorporate this design for recyclability and non-toxicity into other types of polymers to enable a circular lifecycle for some of the most highly used commodity plastics. References: [1] Geyer, R.; Jambeck, J. R.; Law, K. L. Production, Use, and Fate of All Plastics Ever Made. Sci. Adv. 2017, 3 (7). [2] Lithner, D.; Larsson, Å.; Dave, G. Environmental and Health Hazard Ranking and Assessment of Plastic Polymers Based on Chemical Composition. Sci. Total Environ. 2011, 409 (18), 3309–3324. [3] Simón, D.; Borreguero, A. M.; de Lucas, A.; Rodríguez, J. F. Recycling of Polyurethanes from Laboratory to Industry, a Journey towards the Sustainability. Waste Manag. 2018, 76, 147–171. [4] Sternberg, J.; Sequerth, O.; Pilla, S. Green Chemistry Design in Polymers Derived from Lignin: Review and Perspective. Progress in Polymer Science. Elsevier Ltd February 1, 2021, p 101344. [5] Sternberg, J.; Pilla, S. Materials for the Biorefinery: High Bio-Content, Shape Memory Kraft Lignin-Derived Non-Isocyanate Polyurethane Foams Using a Non-Toxic Protocol. Green Chem. 2020. [6] Pilla, Srikanth; Sternberg, J. Non-Isocyanate Polyurethanes from Biobased Polyols. 63/034,584, 2020. [7] Schutyser, W.; Renders, T.; Van den Bosch, S.; Koelewijn, S.-F.; Beckham, G. T.; Sels, B. F. Chemicals from Lignin: An Interplay of Lignocellulose Fractionation, Depolymerisation, and Upgrading. Chem. Soc. Rev. 2018, 47 (3), 852–908. [8] Zhang, K.; Nelson, A. M.; Talley, S. J.; Chen, M.; Margaretta, E.; Hudson, A. G.; Moore, R. B.; Long, T. E. Non-Isocyanate Poly(Amide- Hydroxyurethane)s from Sustainable Resources. Green Chem. 2016, 18 (17), 4667–4681. www.clemson.edu High Pressure Hydrolysis Precipitation / washing Waste NIPU Foram Solubilized Foam Lignin Recovery 28 bioplastics MAGAZINE [01/21] Vol. 16
A new material made from plants and air Strong and flexible foam sheets of biomass plastic for a zero-carbon, circular economy Foam Although PLA is an environmentally-friendly material, it has been difficult to use in foam sheets. Using conventional methods lead to a material that didn’t foam as well as petroleum-derived resins. To address this, the Tokyo headquartered Ricoh Company has utilised supercritical CO 2 in its processing to develop their unique CO 2 fine foam technology. This allowed their experts to create a foamed PLA sheet, known as PLAiR, which is flexible, strong, and environmentally-friendly in ways that were impossible with conventional PLA. Furthermore, PLAiR’s foam expansion rate is adjustable increasing the diversity of applications. PLAiR is a nearly 100 % plant-derived PLA that is flexible yet strong. It undergoes a foaming process utilizing Ricoh’s CO 2 fine foam technology to produce foam sheets. Conventional foam methods cannot be used for processing genuine PLA. To make PLA foam it needs to be blended with other materials, such as fossil-derived resins. The blending process is not the only challenge – bubble sizes are difficult to control and the results are often uneven. As conventional methods can only produce bubbles of large diameters in the order of hundreds of microns the resulting sheets tend to break easily. Beyond that is an inevitable trade-off: adding fossil-derived resins can compromise the carbon-neutrality and biodegradability of PLA. Ricoh’s supercritical CO 2 technology produces uniform bubbles only tens of microns in diameter. Through the kneading process, fillers (foam nucleating agents) are evenly distributed in the material, and the foaming takes place with the fillers as nuclei. In this way, PLA can be made into very thin sheets while its flexibility and strength are maintained because of bubble uniformity. Supercriticality is attained when CO 2 is pressurized at a high temperature. In this state the gas is as dense as liquid, making the CO 2 particles violently collide with each other and produce natural convection. As PLA and fillers are added, they are evenly kneaded using convection. The evenly distributed fillers work as foaming nuclei, resulting in even, minute bubbles. Ricoh is producing PLA foam sheets on prototype machines to promote technological development and verify performance including flexibility, strength, and biodegradability, as well as demonstrate costs. The company intends to deliver the PLA foam sheets in the near future and has plans to expand its partnerships in the thermoformed packaging industry. Ricoh is currently developing PLAiR technology further, to potentially make it into a mass production solution across multiple applications - from cushioning and packing materials to disposable foodware. AT PLA + fossil derived resin + elongation agent Foam particles are large and uneven Cross-section of conventional PLA sheet Supercritical CO 2 kneading process Fillers are evenly distributed Intended purpose Expansion Rate Cushioning and packaging materials High 20 - 25 Food containers Medium 10 - 15 Various trays and containers Low 2 - 3 Thickness 2 -3 mm 1 - 3 mm 0,5 - 1,5 mm Intended purpose Piercing strength (N) JIS Z 1707* Tensile strength (MPa) JIS K 6767* Nano fillers Microbubbles are evenly distributed even thin PLA sheets are flexible Cross-section of Ricoh’s PLAiR Cushioning and packaging materials Food containers Various trays and containers 3,6 2,8 - MD 0,9 1,4 35 TD 0,5 1,1 - Sample table: values shown above are not specifications, but representative sample values (as of November 2020) * JIS: Japanese Industrial Standards JIS K 6767: Cellular plastics – Polyethylene − Methods of test JIS Z 1707: General rules of plastic films for food packaging Info See a video-clip at: https://youtu.be/ dRUQ_FMQaMI Sheet is susceptible to breakage in the narrow sections between bubbles Supercritical CO 2 foaming process Fillers are evenly distributed Micro Foaming https://industry.ricoh.com/en/plair/ bioplastics MAGAZINE [01/21] Vol. 16 29
