Preparation of bio-based rigid polyurethane foam using hydrolytically depolymerized Kraft lignin via direct replacement or oxypropylation
Graphical abstract
Introduction
Rigid polyurethane (PU) foams are widely used for many engineering applications, such as insulation materials, automotive parts, and structural materials [1]. The production of PU foams is realized through the reaction of isocyanates with polyols. Currently, both isocyanate and polyols are mostly derived from petrochemical resources. With the increasing concerns over the depletion of fossil fuels and their environmental impact, there is a growing interest in exploring renewable feedstock to replace petroleum derived polyols either partially or completely for the production of bio-based PU foams [2], without sacrificing the physical, mechanical and thermal characteristics of the material.
Lignin, a natural aromatic high molecular weight biopolymer, is composed of phenyl propanol units [3]. It can be a potential candidate for the production of fuels, chemicals and bio-based materials. All native lignin products are heterogonous in nature and their molecules mainly consist of two types of linkages: condensed linkages (e.g., 5-5 and β-1 linkages) and ether linkages (e.g., α-O-4 and β-O-4) [4]. Aryl ether linkages can be more easily cleaved than the stable C–C linkages which are resistant to chemical depolymerization. Large quantities of lignin are available from pulp and paper mills or cellulosic ethanol plants. It was estimated that the pulp and paper industry generated 50 million tons of lignin in 2010, mainly utilized as a low-value fuel for recovery boilers in pulp/paper mills for heat and power generation [5]. However, only approximately 2% (1 million tonnes) of this lignin was commercialized for other value-added industrial applications. Recently, lignin has gained increasing attention for its potential utilizations, either directly or after chemical modification, as a renewable feedstock for chemicals such as bio-polyols in the preparation of polyurethane (PU) foams [6].
Crude lignin has much lower reactivity [7]; however, due to its larger molecular weight and steric hindrance effect, it can only replace 30% or less of the polyols used in the preparation of PU foams. Hydrolytic depolymerization of Kraft lignin (KL), employing water alone as a solvent under alkaline conditions, was found to be a promising approach to convert high molecular weight KL into low molecular weight depolymerized KL (DKL) [8]. After depolymerization, the resulting DKLs have lower molecular weights and high functionality/hydroxyl numbers [8]. Therefore DKLs could be a substitute for petroleum-based polyols as reactive bio-polyols at a larger replacement ratio than KL for the manufacture of rigid PU foams, one of the most widely used polymeric foam materials, particularly in the construction industry as insulating material.
The objective of this study is to prepare bio-based rigid polyurethane (BRPU) foams with hydrolytically depolymerized KL (DKL) as bio-polyols to substitute petroleum-based polyols at a high replacement ratio (i.e. 50 wt.%). Two preparation routes that differ in the incorporation of DKL into the PU foams were employed in the preparation of BRPU foams: (1) partially replacing PPG400 or sucrose polyol directly with DKL, and (2) oxypropylation of DKL. The obtained BRPU foams were characterized in terms of their physical, mechanical, and thermal properties and compared with reference foams for their potential utilization as insulation materials.
Section snippets
Materials
Kraft lignin (KL) used in this study was provided by FPInnovations, produced using the proprietary LignoForce process [9] in its pilot plant in Thunder Bay, Ontario. The KL has a weight-average molecular weight (Mw ∼10,000 g/mole) based on GPC-UV analysis, and the dried KL sample contains 0.57 wt.% ash and 5.2 wt.% sulfur (on dry ash free basis). Other chemicals used in this work are all CAS reagent grade chemicals, purchased from Sigma–Aldrich and used without further purification, including
Results and discussion
Table 2 summarizes Mw, hydroxyl numbers, and viscosity of the bio-polyols feedstock including the original KL, DKL and oxypropylated samples. As shown in Table 2, both KL and DKL are in solid powder form, while the oxypropylated DKL is in a viscous liquid state (with viscosity of 0.61 Pa s at 80 °C). As shown in Table 2, the DKL and oxypropylated DKL have much higher total hydroxyl numbers (671 mgKOH/g and 350 mgKOH/g, respectively), than the original KL (275 mgKOH/g). In addition to the total
Conclusions
Bio-based rigid polyurethane (BRPU) foams were prepared with depolymerized Kraft lignin (DKL) at high percentage of bio-contents i.e., 50 wt.% via two routes: directly replacing PPG400 or sucrose polyol with DKL, and using oxypropylated DKL as a single polyol feedstock. All foams were characterized in terms of physical, mechanical and thermal properties with their morphology, and their properties were found to be strongly dependent on the DKL incorporation routes. All the foams exhibit good
Acknowledgements
The authors are grateful for the financial support from NSERC/FPInnovations Industrial Research Chair Program in Forest Biorefinery and the Ontario Research Fund-Research Excellence (ORF-RE) from Ministry of Economic Development and Innovation. Support from the industrial partners including FPInnovations, Arclin Canada, BioIndustrial Innovation Centre is also acknowledged.
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