(748e) Lignin-Derived Compounds for the Production of Polyurethane Plastics and Foams

This study investigated the production of polyurethane (PU) materials, wherein the polyol component was partially substituted for lignin. The work established a process for lignin extraction from wood sawdust using a deep eutectic solvent (DES) system, which yielded a lignin of high purity and low polydispersity. A range of lignin based polyols were then prepared, from which PU foams were produced and tested. More than 100 PU foam compositions of varying lignin content were prepared, using a range of different lignin, polyol and isocyanate combinations (see Figure 1). The structural integrity and mechanical properties of each sample were evaluated in order to determine a structure/property relationship between the components utilized and the resulting PU materials.

PUs are one of the most important classes of industrial polymers due to their incredible versatility, ease of use in manufacturing, and low cost. The world market for PU is expected to reach $79 billion by 2021, and flexible PU foams account for 31% of the PU market.[1] The flexible PU market is largely supported by the furniture and automotive industries which use PU foams as upholstery cushioning. The current industry standard for PU foam production is entirely dependent on petrochemical feedstocks for the supply of the major polyol and isocyanate components. This means that the development of a bio-based alternative polyol will not only improve the sustainability of the associated industries, but presents a highly lucrative financial opportunity.

Lignin was pursued for PU production because it is rich in the hydroxyl groups necessary to form urethane linkages, and due to the fact that it is widely available as a waste product from numerous well-established commercial industries. The incorporation of lignin may also lead to PU foams with enhanced physiochemical properties such as biodegradability and ultraviolet light stability.[2]

Materials and Methods

The lignin used in this study was extracted from pine sawdust using a DES comprised of a 2:1 molar ratio of lactic acid and choline chloride. This process is known to produce lignin of very high purity and comparably low polydispersity.[3] The DES lignin was derivatized via oxypropylation in a Parr reactor. This process was conducted in order to increase the accessibility of hydroxyl groups, thus improving the lignin’s reactivity towards the urethane linkage reaction, and the flexibility of the resultant PU material.

The PU foams were produced by first mixing a small amount of additives (catalysts, surfactants and blowing agents) with the prepared lignin polyol mixture. The additives make up less than 0.5% of the mass of the resulting foam. The isocyanate component was then added, and the mixture stirred for several seconds to ensure a homogeneous foam. The reacting mixture was then poured into an open top mold, and allowed to expand upwards. After a curing time of 24 h the foams were removed from their mold. In addition to a qualitative visual inspection, each sample was analyzed in terms of its density, compressive force deflection value (CFDV), and thermal conductivity. A range of samples were also analyzed via scanning electron microscopy (SEM). The results obtained were compared to a control formulation, currently utilized within the automobile industry in upholstery cushion applications.

Results and Discussion

This project combines the societal need to effectively utilize forestry and agricultural wastes, with the commercial interest of producing bio-based materials and plastics from carbohydrates and lignin. By utilizing sawdust, a low-value byproduct of the lumber production process, the lignin feedstock was prepared in a manner which will provide a novel revenue stream to this industry. The lignin was prepared using our research group’s novel, DES extraction procedure. A mild, industrially scalable process, which yields a product of high purity, and more importantly of high structural homogeneity. If lignin is to be successfully applied as a high substitution polyol alternative in semi-rigid foams, these characteristics will be of significant benefit.

PU is mainly used in the form of rigid and semi-rigid foams, and represents a $79bn/year market producing materials for the construction, transportation, furniture, and packaging industries.[1] Utilizing lignin as an alternative low-cost and sustainable resource for PU production represents a great economic opportunity.

Limited by lignin’s rigid aromatic structure and poor isocyanate reactivity, lignin-based polyols have been primarily used for rigid PU foams. Such foams generally exhibit an upper threshold of ~30 wt.% polyol substitution, beyond which the properties of the resultant foams depreciate rapidly.[4] Semi-rigid lignin-based PU foam production has a more lucrative potential, due to semi-rigid foams being produced in significantly greater volume than rigid foams. This field has been previously explored, however current research suggests that the foams become too rigid upon exceeding a lignin content of ~10 wt.%. This study has explored the production of lignin based PU foams using a broad range of compositions, including numerous different lignin types, preparation methods, and degrees of polyol substitution. The work has succeeded in producing PU foams of lignin concentration that meets or exceeds the published literature, whilst maintaining key structural and mechanical characteristics such as density and compressive force deflection value (CFDV).

The results of this project to date confirm the limitation of native lignin as a polyol replacement for semi-rigid PU foam materials. Incorporating underivatized lignins at only 20 wt.% polyol substitution consistently resulted in rigid, structurally inconsistent PU materials. However, when the lignin was oxypropylated prior to its use, it was able to be included at higher degrees of substitution (30 – 40 wt.%), without significantly compromising the mechanical properties of the material.

The oxypropylation approach led to foams with a more consistent cell structure and a higher degree of incorporation of the lignin into the polymer matrix. These foams exhibited higher flexibility and lower density, resulting in materials that more closely emulated the mechanical properties of the commercial formulation.

When preparing composite materials using lignin, a certain degree of increased structural integrity will always be imparted as a result of its aromatic rigid nature. Therefore, in order to produce a PU foam of acceptable rigidity, the type, quantity and ratio of the polyol and isocyanates chosen will have to compensate for this impact. Understanding and controlling the impact of compositional choices will allow for the control of PU foam properties, which will provide opportunities across a range of industrial products. The potential also exists to explore the usage of the residual pulp obtained after DES lignin extraction of the sawdust. One possible application for this byproduct that our group is currently exploring, is as a nanofibril feedstock for the production of alternative Styrofoam packaging materials.

References

1. Gama, N.V., A. Ferreira, and A. Barros-Timmons, Polyurethane Foams: Past, Present, and Future. Materials, 2018. 11(10): p. 1841.
2. Pucciariello, R., et al., Physical properties of straw lignin-based polymer blends. Polymer, 2004. 45(12): p. 4159-4169.
3. Alvarez-Vasco, C., et al., Unique low-molecular-weight lignin with high purity extracted from wood by deep eutectic solvents (DES): a source of lignin for valorization. Green Chemistry, 2016. 18(19): p. 5133-5141.
4. Mahmood, N., et al., Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review. Renewable Sustainable Energy Rev., 2016. 60: p. 317-329.