Performance polyamides built on a sustainable carbohydrate core

The study addresses the challenge of developing sustainable, high-performance plastic precursors that can be produced efficiently from renewable biomass and are compatible with multiple polymer chemistries. Conventional performance polymers often rely on aromatic, petro-derived monomers that are difficult to source sustainably. While bio-based monomers like 2,5-furandicarboxylic acid (FDCA) have enabled high-performance polyesters, FDCA is incompatible with typical melt polyamide synthesis (decarboxylation and low molecular weights), requiring solvent-based or enzymatic routes with limited industrial viability. Succinic acid also struggles to form high-molecular-weight polyamides due to cyclization at high temperatures. Polyamide production presents high global warming potential (GWP), particularly nylon 66, providing a sustainability and economic incentive for bio-based alternatives. The authors propose using dimethyl glyoxylate xylose (DMGX), a stabilized carbohydrate-derived monomer produced efficiently from lignocellulosic feedstocks, as a versatile building block to create amorphous, high-performance polyamides via straightforward, catalyst-free melt polymerization while maintaining competitive properties and enabling improved sustainability metrics.

Prior work highlights FDCA as a top bio-based chemical enabling high-performance polyesters such as poly(ethylene furanoate), but FDCA-based polyamides suffer from decarboxylation under melt conditions, yielding low Mn (<10 kDa), pushing researchers to Yamazaki–Higashi solvent routes, interfacial or enzymatic polymerizations that are not industrially ideal. Succinic acid also fails to produce high-MW polyamides due to intra-cyclization; even direct solid-state polymerization only achieves Mn ~8 kDa. The sector’s sustainability challenge is underscored by nylon 66’s GWP (~8–9 kg CO2e kg−1), with adipic acid being the main contributor. Market prices show opportunities for bio-based competitiveness: nylon 66 at ~US$3–7 kg−1, and semi-aromatic PPAs at ~US$7–20 kg−1. Existing commercial bio-based polyamides (from castor oil C10–C11 chains: PA-11, PA-10,10, PA-10,T) offer low water absorption and flexibility, but have lower mechanical strength and glass transitions than nylon 66. An earlier study demonstrated DMGX could be produced from biomass with high yield and used for degradable polyesters, with a theoretical biomass utilization efficiency (BUE) of 97% and actual BUE around 80%, outperforming bio-based aromatics like terephthalic acid and FDCA in BUE. This background motivates exploring DMGX for polyamides to achieve semi-aromatic-like performance with improved sustainability and processability.

Monomer synthesis: DMGX is produced by acid-catalyzed acetalization of xylose with glyoxylic acid (GA) followed by methanol esterification. The product is a four-stereoisomer mixture that can be purified by extraction, distillation, and crystallization; selective crystallization yields the SS isomer (1S-DMGX) at high purity (>99.6–99.7%). Biomass-to-DMGX yields were previously reported as 95% from purified xylose and 70–83% from lignocellulosic feedstocks, with co-products of stabilized lignin and digestible cellulose, enabling actual BUE up to 80%.

Polyamide synthesis: Poly(alkylene xylosediglyoxylamides) were synthesized via direct melt polycondensation of crystalline DMGX with stoichiometric aliphatic diamines (C6, C8, C10, C12) at 98–100% yield. Reactions used 0.5 wt% triphenyl phosphite (secondary antioxidant) but no polymerization catalyst. Procedure: mixtures heated to 140 °C under N2 with stirring, forming a white solid with methanol distillation; then heated to 250–260 °C, resuming stirring to obtain a viscous melt; vacuum (∼−0.01 to −0.10 mbar) applied to drive polycondensation for 2 h (total 3–4 h reaction). Polymerizations concluded when stirring became inefficient. Products were discharged directly without dissolution/precipitation. Both single-isomer (1S) DMGX and four-isomer (4S) DMGX were used; scale-up to 400 g was demonstrated in a stainless steel polyclave reactor.

Characterization: Molecular weights by SEC-MALS; structure by 1H/13C NMR (and 2D NMR) and MALDI-TOF-MS. Thermal properties by DSC, DMA (with humidity control at 0%, 50%, 100% RH), and TGA under N2. Gas barrier properties to O2 and H2O vapor measured and normalized to 100 µm films. Mechanical properties via tensile testing (ISO 527) on compression- and injection-molded dogbones, in conditioned (23 °C, 50% RH) and dry states. Rheology measured at 250 °C (frequency sweeps) to construct master curves; processability evaluated via piston injection molding, twin-screw extrusion (11 mm co-rotating, L/D 40), filament extrusion, and fused-filament fabrication (3D printing).

Mechanical recycling: 4S PA-10,DGX subjected to three cycles of high-shear twin-screw extrusion at 250 °C; after each cycle, injection-molded specimens were tested for tensile properties. SEC-MALS checked for branching/crosslinking after molding.

Chemical recycling: Methanolysis of 4S PA-8,DGX using 1 M H2SO4 in dry MeOH at 100 °C for ≤4 h. Reaction monitoring by GC-FID with biphenyl internal standard; diamine yield quantified by qNMR in deuterated media. Scaled to 5 g polymer with downstream separations: addition of DCM to precipitate diamine sulfate, filtration, DCM extraction and washing to recover DMGX, recrystallization of DMGX in ethanol, basification and extraction to recover free diamine.

Techno-economic analysis (TEA) and life-cycle analysis (LCA): Aspen Plus simulation (100,000 t y−1 PA-8,DGX pellets) to estimate minimum selling price (MSP) under various diamine and DMGX cost scenarios. LCA (cradle-to-gate) compared PA-8,DGX to PA-6, PA-6,6, PA-6,10, and PA-10,10, considering different DMGX sourcing scenarios (from xylose with fossil GA; from corn cobs with fossil GA; from corn cobs with CO2-derived GA) and sensitivity to 1,8-diaminooctane sourcing.

• Synthesis: Catalyst-free melt polycondensation of DMGX with C6–C12 aliphatic diamines achieved quantitative yields (98–100%) in 3–4 h at 250–260 °C using 0.5 wt% triphenyl phosphite under N2 and vacuum. Successful scale-up to 400 g and use of industrially relevant 4-isomer DMGX mixture were demonstrated.
• Molecular weight: High-MW amorphous polyamides obtained with Mn = 20–30 kDa and dispersities Ð = 1.5–1.8 (4S PA-10,DGX showed Ð ≈ 2.9). 4S vs 1S materials showed comparable MW distributions.
• Thermal properties: Glass transition temperatures (dry) Tg = 114–151 °C (semi-aromatic-like). Humidity significantly reduced Tg: 88–93 °C at 50% RH and 40–60 °C when fully saturated; PA-6,DGX Tg dropped below room temperature in saturated state. TGA under N2 showed 5% mass loss onset at 364–385 °C and Tmax 410–423 °C, indicating high thermal stability. Gas barrier properties were improved over many bio-based plastics but below thin food-packaging requirements.
• Mechanical properties (conditioned, 23 °C, 50% RH, compression-molded): E = 1,700–2,250 MPa; ultimate tensile strength σ = 54–75 MPa; elongation at break ε = 140–165%, with strain hardening. Injection molding increased modulus (e.g., 4S PA-10,DGX E ≈ 2,180 MPa) but reduced ε versus compression molding, consistent with chain alignment. Dry, injection-molded specimens showed σ increase by ~38% (to ~84 MPa), slight E increase (~2,273 MPa), and ε decrease (~18%), similar to behavior in nylon 66.
• Benchmarking: Mechanical properties tuned by diamine length to match aliphatic to semi-aromatic polyamide ranges; PA-8,DGX closely benchmarked an amorphous high-performance semi-aromatic copolyamide (e.g., Grilamid TR 55), suggesting DMGX can substitute isophthalic acid or cycloaliphatic diamines in copolymers.
• Processability: 4S PA-10,DGX exhibited strong shear thinning (complex viscosity ~10^4 Pa·s at 0.1 rad s−1 to ~10^3 Pa·s at 1,000 rad s−1 at 250 °C). Successfully processed by injection molding (260 °C, 1,000 bar), twin-screw extrusion (250 °C, 125 RPM), filament extrusion (1.75 ± 0.1 mm), and 3D printing (nozzle 275 °C, bed 110 °C). Minimal Mn decrease (~5%) after molding.
• Mechanical recycling: After three high-shear extrusion cycles, tensile properties remained nearly unchanged; slight color increase and indications of minor cross-linking (reduced solubility, increased zero-shear viscosity) but reprocessability preserved. Thermal properties largely stable (e.g., ~2 °C decrease in Tg). SEC-MALS showed no significant branching from molding.
• Chemical recycling: Methanolysis at 100 °C with 1 M H2SO4 achieved high monomer recoveries: DMGX up to ~90% by GC and 1,8-diaminooctane ~96% by qNMR. Scaled separations yielded DMGX with 68% overall isolated yield after recrystallization (91% separation yield before recrystallization) and diamine at 61% overall isolated yield; process not fully optimized.
• TEA: For a 100,000 t y−1 plant, MSP of PA-8,DGX lies within the 2022 nylon 66 market price range (US$3–7 kg−1) across diamine cost scenarios and is far below typical PPA prices (US$7–20 kg−1), indicating favorable economics.
• LCA: Compared to nylon 66, PA-8,DGX reduced GWP by ~56% (DMGX from purified xylose + fossil GA) to ~75% (DMGX from corn cobs + fossil GA), and potentially to <1 kg CO2e kg−1 if GA is CO2-derived. Sensitivity to diamine sourcing: worst-case assumption raised GWP to 4.15 kg CO2e kg−1; renewable (fermentation) diamine could lower to ~1.53 kg CO2e kg−1. Other impact categories show typical trade-offs for bio-based routes, mitigated by using agricultural residues rather than dedicated oil crops.

The results demonstrate that a largely intact carbohydrate core (DMGX) can deliver semi-aromatic-like performance in polyamides via a simple, catalyst-free melt polymerization compatible with industrial practice. High Tg (114–151 °C), robust tensile properties, thermal stability, and excellent processability position DMGX-based polyamides as competitive alternatives to fossil-based semi-aromatic polyamides. The materials retain properties over multiple mechanical recycling cycles and allow chemical recycling to monomers with high yields, supporting circularity. From a systems perspective, the high biomass utilization efficiency (theoretical 97%, actual ~80%) stemming from retaining the carbohydrate ring, alongside coproduct valorization (stabilized lignin, cellulose), underpins both sustainability and economic viability. TEA shows MSP comparable to nylon 66, while LCA indicates substantial GWP reductions (up to ~75%) even with fossil GA, and further improvements possible with CO2-derived GA and renewable diamine sourcing. The versatility of DMGX across polyester and polyamide chemistries mirrors the role of phthalates, enabling broad application spaces and economies of scale.

This work establishes DMGX as a versatile, sustainable monomer enabling high-performance amorphous polyamides via straightforward, catalyst-free melt polymerization. The resulting materials achieve semi-aromatic-like thermomechanical properties, are readily processed (injection molding, extrusion, 3D printing), and maintain performance through mechanical recycling, while chemical recycling recovers monomers at high yields. TEA places material costs within nylon 66 market ranges, and LCA projects 56–75% GWP reductions versus nylon 66, with potential for further reductions using CO2-derived GA and renewable diamines. Future work can optimize additive packages to minimize discoloration and suppress cross-linking, improve chemical recycling separations and scale-up yields, explore broader diamine/copolymer formulations (e.g., partial replacement of isophthalic acid or cycloaliphatic diamines), and tailor barrier properties for specific applications.

• Water sensitivity: Tg and mechanical utility decrease with humidity; PA-6,DGX becomes unsuitable in saturated conditions (Tg below room temperature). - Gas barrier performance, while improved over many bio-based plastics, remains below requirements for thin food packaging. - Discoloration during melt polycondensation and slight color increase upon repeated extrusion; may require additive/purification optimization. - Signs of minor cross-linking after high-shear extrusion (reduced solubility, increased zero-shear viscosity), though reprocessability and properties are retained; could limit solvent-based processing (e.g., solvent casting, wet spinning). - Chemical recycling requires relatively harsh acidic conditions (1 M H2SO4 at 100 °C), and current monomer isolation yields, particularly for diamine, are not fully optimized. - LCA sensitivity to diamine sourcing is significant; current analyses assume proxy data (e.g., using hexamethylenediamine or pesticide-related inventories), introducing uncertainty in GWP estimates.