From Science & Research Closing the circle A 100 % plant-based non-isocyanate polyurethane foam capable of chemical recycling The quest for lightweight packaging foams that have a circular lifecycle is a challenging one. Packaging materials and foams account for a large majority of the estimated 400 million tonnes of plastic waste that enters the environment every year [1]. Polyurethanes (PUs) are the sixth most produced plastic and are highly desired for their foaming capabilities as well as their ability to produce materials with properties ranging from elastic to rigid. The problem that many have recognized is that the precursors to PUs, isocyanates, are highly toxic and classified as CMR agents (Cancer-causing, Mutagenic, Reproductive toxins) [2]. Biobased solutions to polyurethanes have largely focused on replacing the polyol portion of the reaction mixture with successes such as the 30 % soy-based PU foam developed by Ford for seat cushions. Replacing the isocyanate portion is much harder since tried-and-true reagents such as toluene diisocyanate and poly(methylenediphenyl) diisocyante are very reactive and allow for the foaming and curing reaction of PUs to occur on industrial timescales. In addition to the issue of toxicity, PUs are also some of the least recycled plastics produced. The crosslinked nature of many PUs precludes the ability to reprocess the material at its end-of-life. While there are some examples of physical recycling where PUs are shredded and rebound in other products, the large-scale conversion of waste PUs to high-value products has not been met with commercial success [3]. Research at the Clemson Composites Centre aimed to address the overarching issues related to the lifecycle of PUs by enabling a non-toxic, biobased, and recyclable PU that could be marketed for high-value applications. To address these needs, the Clemson research team first began by synthesizing a biobased reactive precursor that could be cured and foamed on a timescale used by typical manufacturing practices (~3–5 min). Instead of resorting to biobased vegetable oils that have been used in the past, Kraft lignin was identified as a highly abundant, cheap, and non-edible biobased source for polymer production. Kraft lignin is a by-product of the wood pulping industry and is produced in excess of 70 million tonnes a year [4]. Pulping plants typically use lignin as a fuel source by burning the extracted material in recovery boilers. While many have researched lignin as a feedstock for valuable chemicals and products, it has earned the saying that “you can make anything from lignin, but money” due to its highly crosslinked and heterogeneous structure. Yet, the molecular structure of lignin does contain an abundance of hydroxyl groups that can be utilized to create a more uniform chemical precursor. While propylene or ethylene oxide has been used in the past to create extended etherified chains off the lignin backbone, the harmfulness of these chemicals diminishes the green nature of using lignin as a precursor. Instead, the team found that organic carbonates such as glycerol and dimethyl carbonate could be used to extend hydroxyl groups and create reactive precursors to polyurethane synthesis [5, 6]. The use of glycerol-based chemicals also introduces another biobased source to the synthetic protocol while making use of non-toxic and benign reagents. The use of organic carbonates introduces a functional group (i.e. the cyclocarbonate) that can be cured with aminebased curing agents to create the PU structure. The reaction Tensile Strength of Different Curing Ratios Shape Memory Capacity of Dual-Phase Structure 20 NIPU 1:2 NIPU 1:1.5 1. Increase temp. to 105°C and induce shape 2.Cool to room temp. NIPU 1:1 Stress (MPa) 10 Polymer reverts back to original shape upon re-heating Polymer retains shape set at Temp. > T g 3.Reheat to 105°C under stross-free conditions 0 0,0 0,1 0,2 0,3 0,4 0,5 Strain (mm/mm) 14 bioplastics MAGAZINE [01/21] Vol. 16
By: James Sternberg and Srikanth Pilla Department of Automotive Engineering, Clemson University Clemson, SC, USA Foam of amines and cyclocarbonates creates the urethane bond with no other chemical by-products, another advantageous property of this particular synthetic scheme. However, creating the cyclocarbonated lignin derivative is no easy task. Lignin undergoes radical initiated condensation reactions at elevated temperatures lowering its functionality while creating high molecular weight and insoluble precursors [7]. To use lignin for polymer synthesis, it was found that precise time, temperature, catalyst-loading, and reagent concentrations were necessary. However, finding these conditions allowed for a predictable reaction at nearly quantitative yields. Curing with diamines may not sound like the most environmentally favourable solution, yet a fatty-acid-based dimer diamine with 100 % renewable carbon was sourced, that had an LD50 value above 5000 mg/kg, essentially placing it in the category of nontoxicity. While some risk is always associated with diamines, this solution is a great step beyond the use of isocyanates. NIPU / Foam % Density (kg/m 3) Compressive Strength (10 %, kPa) Compressive Modulus (MPa) 1:1 / 3 % 241 ± 34 131,8 ± 37,9 1,42 ± 0,22 1:1 / 1,5 % 337 ± 57 170,0 ± 18,6 1,64 ± 0,32 1:2 / 3 % 241 ± 45 79,2 ± 18,5 1,34 ± 0,25 1:2 / 1,5 % 331 ± 39 111,9 ± 28 1,19 ± 0,30 The reactivity of the novel formulation was tested by monitoring the gel time between lignin precursors and the diamine curing agent. While visual evidence showed a clear gelation of the mixture around 5 minutes, rheological analysis was able to show more precisely that the reaction mixture began to take on solid-like characteristics at around 3 minutes with a curing temperature of 80°C. This allowed the addition of a chemical foaming agent, poly(methylhydrosiloxane) to create the cellular structure of the foams. A curing study showed that a temperature of 150°C was necessary to create fully cured resins showing the highest mechanical properties. However, curing at lower temperatures does enable a more flexible material by lowering the crosslinking density of the non-isocyanate polyurethane (NIPU). The biobased dimer diamine was successful at adding soft segments throughout the NIPU structure, tempering the brittle nature of the lignin backbone. It was found that along with curing temperature, increasing the ratio of diamine also created samples with higher ultimate strain values. The lignin-derived NIPU recorded the highest tensile strength of any reported NIPU in its class, demonstrating ultimate stress values above 20 MPa. With the foamed samples, a benchmark for rigid foams set at 100 kPa for 10 % deflection was targeted, a mark the foams easily surpassed. In terms of thermal stability, the NIPU recorded 5 % weight loss temperatures at 300°C, an improvement above many commercial polyurethanes [8]. Mechanical Properties of Foams SEM of Lowest Density Foam bioplastics MAGAZINE [01/21] Vol. 16 15
