Studying the Suitability of Nineteen Lignins as Partial Polyol Replacement in Rigid Polyurethane/Polyisocyanurate Foam

Abstract

In this study, nineteen unmodified lignins from various sources (hardwood, softwood, wheat straw, and corn stover) and isolation processes (kraft, soda, organosolv, sulfite, and enzymatic hydrolysis) were used to replace 30 wt.% of petroleum-based polyol in rigid polyurethane/polyisocyanurate (PUR/PIR) foam formulations. Lignin samples were characterized by measuring their ash content, hydroxyl content (Phosphorus Nuclear Magnetic Resonance Spectroscopy), impurities (Inductively Coupled Plasma), and pH. After foam formulation, properties of lignin-based foams were evaluated and compared with a control foam (with no lignin) via cell morphology, closed-cell content, compression strength, apparent density, thermal conductivity, and color analysis. Lignin-based foams passed all measured standard specifications required by ASTM International C1029-15 for type 1 rigid insulation foams, except for three foams. These three foams had poor compressive strengths, significantly larger cell sizes, darker color, lower closed-cell contents, and slower foaming times. The foam made with corn stover enzymatic hydrolysis lignin showed no significant difference from the control foam in terms of compressive strength and outperformed all other lignin-based foams due to its higher aliphatic and p-hydroxyphenyl hydroxyl contents. Lignin-based foams that passed all required performance testing were made with lignins having higher pH, potassium, sodium, calcium, magnesium, and aliphatic/p-hydroxyphenyl hydroxyl group contents than those that failed.

Lignin-Based Low-Density Rigid Polyurethane/Polyisocyanurate Foams

“…Although the addition of lignin into the foam increased reaction times, all the foams had acceptable reaction times for rigid foam synthesis. , Our previous study was the only other study that incorporated unmodified lignin into low-density rigid PUR/PIR foams. We used 19 technical lignins to replace 30 wt % of the petrochemical polyol with lignin and found that foams with higher reaction times (∼357 s) had significantly worse compression strengths, closed cell contents, and cell morphologies . All control and lignin-based foams were well below this reaction time by 27–75%.…”

“…After the foaming reaction characterization, foams were evaluated for apparent density (Table ). The apparent density of rigid foams is known to have a logarithmic correlation affecting most other foam properties outside the 30–60 kg/m 3 low-density range. ,,, The densities of lignin-based foams ranged from 41 to 54 kg/m 3 (Table ), falling within the low-density foam range, making the effect of density on foam properties insignificant. The increased density, compared to the control (35 kg/m 3 ), in lignin-based foams is likely caused by the use of solid lignin, which can delay foaming times (Figure ), increase polyol blend viscosity (Table ), and increase the degree of foam crosslinking. − ,, Even so, all lignin-based foams had suitable apparent densities for rigid insulation and structural applications indicated by ASTM D7425 and E1730. , …”

“…Compression strength has also been reported as a limiting factor for lignin incorporation, along with decreased foam reactivity in lignin-based foams. , Due to foam anisotropy, the compression strength of foams almost doubles based on testing samples perpendicular to foam rise compared to parallel. , Here (Figure ), we evaluate compression strength perpendicular to foam rise to ensure we meet minimum requirements on the weakest side of the foam. The highest compression strength of control and lignin-based foams was obtained at 50% lignin loading (Figure ).…”

“…Cyclohexanol solution consisting of anhydrous pyridine and deuterated chloroform was then added as an internal standard. ,, Chromium (III) acetylacetonate solution (anhydrous pyridine and deuterated chloroform) was added as a relaxation reagent along with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane as phosphitylation reagent. Spectra were acquired using an Agilent DDR2 500 MHz NMR spectrometer (Billerica, MA, USA) . The hydroxyl value of lignin was then calculated by multiplying the measured total hydroxyl content of lignin by the mass of KOH (56.1) …”

“…Apparent density was determined according to ASTM D1622 following a previously published method . Each sample was measured volumetrically to 0.01 significant figures (three times per side) using a digital caliper and weighed using a digital scale (0.0001).…”