Chemi-Thermomechanical Pulping of Wheat Straw, Miscanthus, and Bamboo: Fiberization Energy, Morphology, and Containerboard-Relevant Properties

Non-wood feedstocks represent alternative fiber sources for high-yield pulping, yet their processing behavior and papermaking performance relative to industrial benchmarks remain insufficiently characterized. This study evaluates chemi-thermomechanical pulping (CTMP) of three non-wood feedstocks, wheat straw ( Triticum aestivum ), miscanthus ( Miscanthus × giganteus ), and bamboo ( Phyllostachys nigra ‘Henon’), alongside eucalyptus ( Eucalyptus urograndis ) and industrial benchmark pulps including unbleached softwood kraft (USK), old corrugated containers (OCC), and semi-chemical hardwood (SCHW). Under mild pre-treatment conditions (4% Na 2 SO 3 , 4% NaOH, 90–100 °C, and 60 min), non-wood feedstocks (280–393 kW·h/t) required 27%–48% less fiberization energy than eucalyptus (538 kW·h/t), with rejects below 6% for all furnishes. Pulping yields ranged from 72.2 % to 86.0 % for non-wood feedstocks and 82.6 % to 90.0 % for eucalyptus. Non-wood CTMP pulps reached target freeness with fewer Papirindustriens Forskningsinstitutt (PFI) mill revolutions than USK; bamboo and wheat straw developed sheet densities above 500 kg/m 3 at 500 mL Canadian standard freeness (CSF), while miscanthus and eucalyptus CTMP remained below 450 kg/m 3 . At 500 mL CSF, bamboo CTMP exhibited the highest burst strength among all furnishes (3.33 kPa·m 2 /g), exceeding USK (2.65 kPa·m 2 /g) and OCC (2.73 kPa·m 2 /g). The tensile index of bamboo (40.4 N·m/g) and wheat straw (36.4 N·m/g) surpassed SCHW (32.0 N·m/g), while miscanthus (19.0 N·m/g) remained below it; the USK retained the highest tensile index, short-span compressive strength (STFI), and stiffness index across all furnishes. Eucalyptus CTMP at 90% yield recorded the lowest strength values in the dataset. Fiber morphology analysis showed that bamboo’s lowest coarseness (118 mg/km) and highest slenderness ratio (78.7) among CTMP furnishes arose from narrow fiber width rather than length, while miscanthus exhibited the highest Runkel ratio (0.543) and coarseness (331 mg/km), indices consistent with the mechanical performance hierarchy observed. Overall, bamboo and wheat straw CTMP showed the greatest potential for strength-oriented applications, while miscanthus exhibited lower strength development despite its low fiberization-energy requirement. These findings demonstrate the potential of non-wood feedstocks for CTMP production while highlighting the need to address feedstock-specific contaminants, storage, handling, and process-integration challenges before industrial-scale implementation.