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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Integration of polyurethane meniscus scaffold during ACL revision is not reliable at 5 years despite favourable clinical outcome

Abstract Purpose The aim of this study was to evaluate the clinical outcome at 5‐year follow‐up of a one‐step procedure combining anterior cruciate ligament (ACL) reconstruction and partial meniscus replacement using a polyurethane scaffold for the treatment of symptomatic patients with previously failed ACL reconstruction and partial medial meniscectomy. Moreover, the implanted scaffolds have been evaluated by MRI protocol in terms of morphology, volume, and signal intensity. Methods Twenty patients with symptomatic knee laxity after failed ACL reconstruction and partial medial meniscectomy underwent ACL revision combined with polyurethane‐based meniscal scaffold implant. Clinical assessment at 2‐ and 5‐year follow‐ups included VAS, Tegner Activity Score, International Knee Documentation Committee (IKDC), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and the Lysholm Score. MRI evaluation of the scaffold was performed according to the Genovese scale with quantification of the scaffold’s volume at 1‐ and 5‐year follow‐ups. Results All scores revealed clinical improvement as compared with the preoperative values at the 2‐ and 5‐year follow‐ups. However, a slight, but significant reduction of scores was observed between 2 and 5 years. Concerning the MRI assessment, a significant reduction of the scaffold’s volume was observed between 1 and 5 years. Genovese Morphology classification at 5 years included two complete resorptions (Type 3) and all the remaining patients had irregular morphology (Type 2). With regard to the Genovese Signal at the 5‐year follow‐up, three were classified as markedly hyperintense (Type 1), 15 as slightly hyperintense (Type 2), and two as isointense (Type 1). Conclusion Simultaneous ACL reconstruction and partial meniscus replacement using a polyurethane scaffold provides favourable clinical outcomes in the treatment of symptomatic patients with previously failed ACL reconstruction and partial medial meniscectomy at 5 years. However, MRI evaluation suggests that integration of the scaffold is not consistent. Level of evidence Level IV.

Pereira, Hélder↗
DOE OSTI2024

A chemo-mechanical model for describing sorption hysteresis in a glassy polyurethane

Abstract Hysteretic sorption and desorption of water is observed from 0 to 95% relative humidity and 298–333 K on a glassy polyurethane foam. It is postulated that sorption-induced swelling of the glassy polyurethane increases the concentration of accessible hydrogen-bonding adsorption sites for water. The accessibility of sites is kinetically controlled due to the restricted thermal motions of chains in the glassy polymer, causing a difference in accessible site concentrations during sorption and desorption. This discrepancy leads to hysteresis in the sorbed concentrations of water. A coupled chemo-mechanical model relating volumetric strain, adsorption site concentration, and sorbed water concentration is employed to describe water sorption hysteresis in the glassy polyurethane. This model not only describes the final mass uptake for each relative humidity step, but also captures the dynamics of water uptake, which exhibit diffusion and relaxation rate-controlled regimes.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Flexible polyurethanes, renewable fuels, and flavorings from a microalgae oil waste stream

Renewable polymers have become an important focus in next-generation materials, and algae biomass offers an environmentally low-impact feedstock that can serve multiple uses. This study aims to develop a scalable methodology for production of microalgae-based polyols for polyurethane synthesis from waste oils derived from algae biomass. Following separation of omega-3 fatty acids from algae oil, residual oils can offer valuable building blocks for petrochemical replacements. However, unlike vegetable oils, algae oils contain organic contaminants, including photosynthetic pigments and hydrophobic cofactors that can complicate preparative methodologies. Here we convert and purify waste streams from omega-3 depleted Nannochloropsis salina algae oil, with major components consisting of palmitic and palmitoleic acid, into azelaic acid (AA) as a building block for flexible polyurethanes, with a simultaneous production of heptanoic acid (HA) as a flavor and fragrance precursor. Conversion of free fatty acid mixtures into a soft soap allows extraction of organic contaminants, and urea complexation provides isolated palmitoleic acid, which is subsequently ozonolyzed to produce AA and HA. Bio-based polyester diols are prepared from AA via esterification to provide a polyol monomer for flexible polyurethane foam preparation. The HA co-product is modified to produce the flavoring agent methyl heptanoate and also decarboxylated to produce hexane as a renewable solvent. This scalable process can be performed on oils from multiple algal species, offering valuable monomers from a highly sustainable source.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials for the biorefinery: high bio-content, shape memory Kraft lignin-derived non-isocyanate polyurethane foams using a non-toxic protocol

Polyurethanes are among the top six polymers produced in the world and are widely used in the automotive, furniture, construction and appliance industry for their light weight, impact resistance, and insulating properties. However, the use of hazardous diisocyanates used in polyurethane formulations has led many to search for more sustainable alternatives. The lignin component of biomass has been targeted to replace the often toxic and petroleum-derived precursors to polymer synthesis in support of the biorefinery concept where natural materials are used as the feedstock for commodity plastics. The use of lignin is often hampered due to its low reactivity, heterogeneity and the necessity of employing extensive purification and/or functionalization measures to ensure materials are of comparable quality. In this report, a unique method is presented for the synthesis and processing of a non-toxic, non-isocyanate polyurethane foam utilizing unmodified Kraft lignin and a biobased curing agent. Here, raw Kraft lignin is functionalized with green organic carbonates and a non-toxic approach is taken to solubilize the precursors with a curing agent from renewable fatty acids enabling rapid gel-times. For the first time, a NIPU foam from lignin is synthesized while the structure–property relationship of different reaction mixtures is studied demonstrating shape memory capacity and 100% biobased carbon content.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Design of closed-loop recycling production of a Diels–Alder polymer from a biomass-derived difuran as a functional additive for polyurethanes

Acetalization of biomass-derived 5-hydroxymethyl furfural (HMF) with pentaerythritol produced a difuran (HPH) monomer in the presence of an acid catalyst. A recyclable polymer was then synthesized by Diels–Alder reaction of bismaleimide and the HMF-derived difuran (HPH). A polyurethane, produced from the Diels–Alder polymer has a higher glass transition temperature than a polyurethane, produced from ethylene glycol. The polyurethane, containing Diels–Alder polymer also has a self-healing ability. The Diels–Alder polymer could be hydrolyzed in acidic acetate buffer at 60 °C to produce the monomers for recycling. Each produced monomer was separated by solvent extraction, and the extracted monomers were recovered in different solvent fractions, such as aqueous, ethyl acetate, and acetone fractions. Techno economic analysis was used to assess the minimum selling price ($\$14.1$ per kg) for the primary production of Diels–Alder polymer at a feed capacity of 400 tons per year. The economic viability of the primary recovery process for the most expensive recovered monomer, bismaleimide, was assessed by calculating the minimum selling price of the bismaleimide ($\$15.2$ per kg). In conclusion, a circular closed-loop recycling production process for the Diels–Alder polymer was developed and this approach can produce the Diels–Alder polymer at $8.2 per kg when the feed capacity was 40 ktons per year.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Enhanced Production of Algae Lipids and Carbohydrates for Fuel and Polyurethane Precursors

This Final Technical Report summarizes the goals, approach, and outcomes of the project “Enhanced Production of Algae Lipids and Carbohydrates for Fuel and Polyurethane Precursors”. The project addressed the challenge of improving microalgae biomass productivity and simultaneously producing valuable polyurethane precursors (PUPs), from both lipids and carbohydrates, that can be converted into renewable biofuels and bio-based polyurethane (PU) products. By integrating advanced genetic engineering, traditional breeding, high-throughput screening, pilot cultivation, and chemical conversion technologies, the team achieved significant advancements in algae biotechnology.

09 BIOMASS FUELS↗
DOE OSTI2025

Laboratory Testing to Assess the Feasibility of Polyurethane Flat Belts for Marine Energy Applications

Polyurethane flat belts have received limited scientific attention as load-bearing elements in marine energy systems, particularly in applications involving dynamic tensile and bending loads. This study evaluates their potential as a replacement for traditional wire ropes in marine energy applications, with a focus on their ability to be integrated into winch-driven wave energy converters where bending and tensile stresses can make long-term operation difficult. Polyurethane belts are hypothesized to offer enhanced fatigue resistance due to their reduced thickness in the bending plane and therefore lower bending stresses. This research involves a series of tests utilizing the National Renewable Energy Laboratory’s (NREL) Large-Amplitude Motion Platform to replicate the dynamic conditions experienced by mooring lines of winch-based point-absorber-type marine energy converters. The conditions tested include unequal coiling and uncoiling tensions and load cases resulting from the device’s unconstrained movement relative to its anchor, such as twisting and off-axis loading. Results from this study show that polyurethane flat belts can achieve more than 198 percent of the fatigue life of a conventional wire rope under similar load profiles. The stress concentrations resulting from off-axis loading and cumulative twist beyond the system’s allowable limits have been identified as potential failure modes for flat belt mooring lines used in winch-driven wave energy converters deployed in ocean environments. To mitigate these risks, the use of anti-spin systems and fairleads designed to accommodate off-axis loading while limiting twist accumulation is recommended.

13 HYDRO ENERGY↗
NASA NTRS1985

Polyurethane Masks Large Areas in Electroplating

Polyurethane foam provides effective mask in electroplating of copper or nickel. Thin layer of Turco maskant painted on area to be masked: Layer ensures polyurethane foam removed easily after served its purpose. Component A, isocyanate, and component B, polyol, mixed together and brushed or sprayed on mask area. Mixture reacts, yielding polyurethane foam. Foam prevents deposition of nickel or copper on covered area. New method saves time, increases productivity and uses less material than older procedures.

Beasley, J. L.↗
NASA NTRS2000

Uncertainty Analysis of Decomposing Polyurethane Foam

Sensitivity/uncertainty analyses are necessary to determine where to allocate resources for improved predictions in support of our nation's nuclear safety mission. Yet, sensitivity/uncertainty analyses are not commonly performed on complex combustion models because the calculations are time consuming, CPU intensive, nontrivial exercises that can lead to deceptive results. To illustrate these ideas, a variety of sensitivity/uncertainty analyses were used to determine the uncertainty associated with thermal decomposition of polyurethane foam exposed to high radiative flux boundary conditions. The polyurethane used in this study is a rigid closed-cell foam used as an encapsulant. Related polyurethane binders such as Estane are used in many energetic materials of interest to the JANNAF community. The complex, finite element foam decomposition model used in this study has 25 input parameters that include chemistry, polymer structure, and thermophysical properties. The response variable was selected as the steady-state decomposition front velocity calculated as the derivative of the decomposition front location versus time. An analytical mean value sensitivity/uncertainty (MV) analysis was used to determine the standard deviation by taking numerical derivatives of the response variable with respect to each of the 25 input parameters. Since the response variable is also a derivative, the standard deviation was essentially determined from a second derivative that was extremely sensitive to numerical noise. To minimize the numerical noise, 50-micrometer element dimensions and approximately 1-msec time steps were required to obtain stable uncertainty results. As an alternative method to determine the uncertainty and sensitivity in the decomposition front velocity, surrogate response surfaces were generated for use with a constrained Latin Hypercube Sampling (LHS) technique. Two surrogate response surfaces were investigated: 1) a linear surrogate response surface (LIN) and 2) a quadratic response surface (QUAD). The LHS techniques do not require derivatives of the response variable and are subsequently relatively insensitive to numerical noise. To compare the LIN and QUAD methods to the MV method, a direct LHS analysis (DLHS) was performed using the full grid and timestep resolved finite element model. The surrogate response models (LIN and QUAD) are shown to give acceptable values of the mean and standard deviation when compared to the fully converged DLHS model.

Hobbs, Michael L.↗
NASA NTRS2006

Fracture Toughness Evaluation of Space Shuttle External Tank Thermal Protection System Polyurethane Foam Insulation Materials

Experimental evaluation of the basic fracture properties of Thermal Protection System (TPS) polyurethane foam insulation materials was conducted to validate the methodology used in estimating critical defect sizes in TPS applications on the Space Shuttle External Fuel Tank. The polyurethane foam found on the External Tank (ET) is manufactured by mixing liquid constituents and allowing them to react and expand upwards - a process which creates component cells that are generally elongated in the foam rise direction and gives rise to mechanical anisotropy. Similarly, the application of successive foam layers to the ET produces cohesive foam interfaces (knitlines) which may lead to local variations in mechanical properties. This study reports the fracture toughness of BX-265, NCFI 24-124, and PDL-1034 closed-cell polyurethane foam as a function of ambient and cryogenic temperatures and knitline/cellular orientation at ambient pressure.

McGill, Preston↗
NASA NTRS2011

Potential Alternatives Report for Validation of Alternatives to Aliphatic Isocyanate Polyurethanes

Identifying and selecting alternative materials and technologies that have the potential to reduce the identified HazMats and hazardous air pollutants (HAPs), while incorporating sound corrosion prevention and control technologies, is a complicated task due to the fast pace at which new technologies emerge and rules change. The alternatives are identified through literature searches, electronic database and Internet searches, surveys, and/or personal and professional contacts. Available test data was then compiled on the proposed alternatives to determine if the materials meet the test objectives or if further)laboratory or field-testing will be required. After reviewing technical information documented in the PAR, government representatives, technical representatives from the affected facilities, and other stakeholders involved in the process will select the list of viable alternative coatings for consideration and testing under the project's Joint Test Protocol entitled Joint Test Protocol for Validation of Alternatives to Aliphatic Isocyanate Polyurethanes and Field Test Plan entitled Field Evaluations Test Plan for Validation of Alternatives to Aliphatic Isocyanate Polyurethanes, both prepared by ITB. Test results will be reported in a Joint Test Report upon completion oftesting. The selection rationale and conclusions are documented in this PAR. A cost benefit analysis will be prepared to quantify the estimated capital and process costs of coating alternatives and cost savings relative to the current coating processes, however, some initial cost data has been included in this PAR. For this coatings project, isocyanates, as found in aliphatic isocyanate polyurethanes, were identified as the target HazMat to be eliminated. Table 1-1 lists the target HazMats, the related process and application, current specifications, and affected programs.

Lewis, pattie↗
NASA NTRSDate not supplied

Auxetic Thermo-Responsive Shape-Memory Polyurethane Aerogels

Shape memory polymer aerogels are stimuli-responsive aerogel materials that incorporate polymer aerogel characteristics such as flexibility at low density, and high porosity with shape-changing capabilities upon exposure to external stimuli. By designing the structure of these materials at meso- to macroscale, their shape memory response can be efficiently engineered to show an auxetic response such as a negative Poisson’s ratio, leading to an active and tunable metamaterial. We present a material technology based on a shape memory polyurethane aerogel derived from aliphatic triisocyanate and triethylene glycol, which demonstrates and remembers an auxetic shape change through a thermomechanical cycle first below, and then back again above the glass transition temperature of the polyurethane. Our auxetic shape memory polyurethane aerogels exhibit a significant auxetic response with a negative Poisson’s ratio of approximately -0.8 at 15% compressive strain. This technology is expected to impact multiple areas of interest in relation to commercial, aeronautic, and aerospace industries. Potential applications include minimally invasive medical technologies, soft robotics, and self-deployable structures and habitats. Structure-property relationships of the auxetic shape memory polymer aerogels will be discussed.

Sadeq Malakooti↗
DOE OSTI2020

Characterizing The Mechanical Properties of Polyurethane Foams

The Department of Energy has many radioactive facilities that are on the path of deactivation and decommissioning (D and D). These facilities can sit cold and dark for many years before final disposition, and must be maintained to ensure no radiological release occurs in the interim. Improvements and additions to D and D tool sets can greatly: Save time and money, Reduce worker risk. Conventional fixatives widely used often take the form of paints or films that are not readily applicable to 3-dimensional void spaces. Foams are one promising platform that may offer solutions to a number of contaminated problem sets such as: Gloveboxes, Pipes, Tanks. SRNL is working to characterize various commercial foams that would encapsulate the interior volume of a given space and are capable of immobilizing any remaining contamination. One key performance metric of these foams is how well they will perform in an accident scenario. To this end, SRNL is researching the mechanical properties of these foams to ensure that the material can withstand the environment of application while maintaining structural integrity. ASTM standard E3191-18 served as a guiding document for this project, outlining the requirements that foaming fixatives used for the mitigation of radioactive contamination need to meet before being implemented. Objective: Quantification of the mechanical properties of 6 commercial polyurethane foams was performed using multiple ASTM standards to record measurements for: Compression testing: Flexible Foams, Force required to produce 50% compression, Rigid Foams, Compressive and apparent modulus, Point of 10% core deformation, The 'Zero Deformation' point, Compressive strength, Yield point. Tensile testing: Tensile strength, Tensile stress, Percent elongation. Experiment 1: An electromechanical compressive tester (MTS Criterion Series 43) was used to evaluate 6 foams (4 flexible, 2 rigid). Per ASTM D1621, each rigid foam was compressed at 10% of the measured thickness per minute until the sample was 13% of it's original thickness. Per ASTM D35/4, each flexible foam was pre-flexed twice to 80% original thickness at a rate of 250 mm/min, then compressed to 80% original thickness at a rate of 50 mm/min. Experiment 2: An electromechanical tensile tester (MTS Criterion Series 43) was used to evaluate 6 foams (4 flexible, 2 rigid). Per ASTM D1623, rigid foams were pulled apart at a rate of 1.27 mm/min until the sample broke. Per ASTM D35/4, flexible foams were pulled apart at a rate of 500 mm/min until the sample broke. The strongest material in both compression and tensile testing scenarios was found to be the rigid intumescent polyurethane Hilti foam. The experiments revealed that the Hilti foam in a tensile scenario had a peak stress value that was larger than the closest competitor by a factor of 2.3 and a compressive yield point that was larger than the closest competitor by a factor of 1.4, indicating that the Hilti foam is the best choice for implementation in mechanically harsh environments. The performance metrics measured can serve as a basis for future mechanical tests that would help set relevant ASTM standards (E3191) for intumescent polyurethane foams in fixating applications. Tests like surface adhesion, impact, and flame tests would serve as a better indicator as to how this material would perform in environmentally harsh scenarios often found in decommissioned nuclear facilities. Further tests of the foams' intumescent properties would also be important should these foams be implemented in environmentally harsh scenarios.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2024

Structure Development in Cross-Linked, Soybean Oil-based Waterborne Polyurethanes

Development of waterborne polyurethanes (WPU) using bio-based sources represents a step towards sustainable materials science and industry. We synthesized bio-based cationic water-dispersed crosslinked polyurethanes from high oleic soybean oil (HOSO) polyol, isophorone diisocyanate, and methyldiethanol amine, with varying ionic group contents after neutralization with acetic acid. Our primary objective was to analyze how crosslinking affects the dispersion process and film properties in multifunctional systems. The synthesis-structure-property relationship is elucidated through comprehensive analyses of the products at different stages of the synthesis. The dispersion of the WPU particles in water must occur prior to gelation during the final preparation, leading to incomplete conversion and the formation of imperfect networks. Insight into the synthesis process and polymer structure was gained by simulating polymer network parameters. Morphological analyses using synchrotron-based X-ray scattering and atomic force microscopy revealed a hierarchical structure within the WPU films. Importantly, all the films prepared in this study, without using coalescence agents, have low water absorption and high water contact angles, demonstrating their potential for textile and leather coatings and other applications.

bio-based polymers↗
DOE OSTI2023

Biodegradable waterborne polyurethane-urea dispersion adhesives with high biocontent

Non-biodegradable petroleum-based plastic wastes have become a leading environmental concern, and new efforts are underway to prepare biobased and biodegradable replacements. For this work, we have explored the preparation of adhesives suitable for use in consumer products, and here we report the development of waterborne, biodegradable adhesives from biobased monomers resulting in adhesives exceeding 70% biocontent. Using water as the polymer medium, viscosity challenges and the use of volatile organic solvents are avoided. Material properties of the polyurethane dispersions, resulting films, and laminates produced showed M w ranging between 56,000 and 124,000. Lastly, the biodegradability of films and laminates was evaluated. The resulting metrics indicate that the adhesives produced meet the desired mechanical and biodegradability targets, indicating that high renewability content solvent-free polyurethane dispersions are a viable solution for lamination adhesives.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Kinetic Monte Carlo Tool for Kinetic Modeling of Linear Step-Growth Polymerization: Insight into Recycling of Polyurethanes

A kinetic Monte Carlo model of polyurethane polymerization which explicitly tracks the polymer sequences is developed and shared. Here this model is benchmarked against theoretical and experimental polyurethane data and used to investigate the effect on oligomer distributions of unequal reactivity of the first and second isocyanate to react. The reverse reactions using thermodynamic consistency are then added to the framework, and analogous to the addition polymerization concept of ceiling temperature, equilibrium chain length distributions at various temperatures are calculated. For a mixture of three monomers AA, BB, and CC, where BB and CC do not react with one another, are present in stoichiometric proportions, and have different enthalpies of reaction with AA, an odd-even effect emerges. Odd length chains are more likely than even length chains for temperatures at which BB and CC have significantly different equilibrium conversions. The concept of ceiling temperature that is typically cited for addition polymers is extended here to provide a measure of conditions under which depolymerization for recycling is favored.

condensation polymerization↗
DOE OSTI2024

Green co-solvent-assisted one-pot synthesis of high-performance flexible lignin polyurethane foam

Although lignin is a promising candidate as a source of polyol for producing eco-friendly polyurethane foam (PUF), its direct application has limitations, primarily stemming from the brittleness of the resulting products and poor processibility at high lignin loadings. To address these technical challenges, we introduced an effective co-solvent for the lignin-based polyurethane foams (LPUFs) with up to 80wt% of lignin substitution for petroleum polyols. As a green co-solvent that involves carbon dioxide fixation during its production, propylene carbonate (PC), was blended with lignin and improved the distribution of lignin by decreasing the viscosity, providing a room-temperature reaction. The LPUFs demonstrated a comparable thermal insulation performance (R-values greater than 5 in -1 ) with commercially available PUF with a uniform size of closed-cell structure induced by the reduced viscosity of lignin-based polyol. LPUFs also showed enhanced fire retardancy and moisture resistance compared to the control PUFs, benefiting from lignin’s intrinsic fire-resistant and hydrophobic properties. To mitigate the brittleness issue at high lignin loadings, flexible polyethylene glycols (PEGs) with a high molecular weight (i.e., PEG-1000) were dissolved in PC at room temperature to further improve the mechanical strength of LPUFs. The compressive strength of LPUFs with 80 wt% lignin substitution content was remarkably improved to 226 kPa by the employment of co-solvent and PEG-1000 compared to the control PUF (<140 kPa). Moreover, the flexibility of the LPUFs was successfully controlled by simply adjusting the PEG-400/PEG-1000 ratio. In conclusion, the results of this study provide valuable insights for accelerating and broadening the utilization of lignin in sustainable materials and manufacturing areas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

From acetone fractionation to lignin-based phenolic and polyurethane resins

In this paper, acetone fractionation was used to increase the homogeneity of softwood kraft lignin for product development. The acetone soluble and insoluble fractions were analyzed in terms of elemental composition, chemical structure, and thermal properties. The phenolic hydroxyl content of the acetone soluble fraction increased by 24%. The acetone soluble fraction of kraft lignin was used to fully substitute phenol in phenolic resin formulation. The resulting formulated lignin-formaldehyde adhesive showed higher adhesion strength (4.3 MPa) compared to a lab formulated phenol-formaldehyde resin (3.4 MPa) with 100% wood failure after lap shear test. On the other hand, the acetone insoluble lignin fraction, which had 23% higher aliphatic hydroxyl content compared to the starting lignin, was used to entirely replace petroleum-based polyol in polyurethane resin formulation. Then a bio-derived solvent (dihydrolevoglucosenone, Cyrene) was used to dissolve the acetone insoluble lignin fraction to synthesize lignin-based polyurethane resin. Tailoring the structure of kraft lignin through solvent fractionation resulted in two fractions, which were used to replace 100% of two major petrochemicals (phenol and polyol) in the formulations of phenolic and PU resins, that would otherwise be challenging due to the heterogeneity of softwood kraft lignin.

36 MATERIALS SCIENCE↗