Valorization of post-industrial cotton waste for cellulose nanocrystal production via sulfuric and mixed acid hydrolysis

In this study, the starting material for the CNC production was post-industrial cotton waste from continuous tumbler dryer operations. Hydrolysis systems were studied at fixed total acid concentration of 64 wt%, including unary sulfuric acid (SA) and three binary combinations: SA:phosphoric acid (SA:PA), SA:oxalic acid (SA:OA) and SA:acetic acid (SA:AA) at varying temperature and time conditions. Pure SA had a limited performance, with a maximum yield of 7.51%, as prolonged hydrolysis under strong acid leads to nonselective chain scission which is detrimental to recovery. Binary systems demonstrated markedly superior performance. SA:OA returned 88.92% at 50 °C/90 min; SA:PA reached 87.65% under similar conditions. Introducing a weaker secondary acid appears to attenuate proton activity enough to favor amorphous region cleavage without attacking the crystalline fraction. Structural characterization compared SA and SA:PA products against commercial CNC. Crystallinity by XRD was 84% and 85% for the synthesized samples versus 74% for commercial CNC. FTIR confirmed preservation of cellulose functional groups. Rod-like morphology was confirmed by TEM in both cases, though dimensions differed considerably—SA:PA produced wider, longer crystals than SA, consistent with phosphoric acid slowing lateral fibril separation. SA:PA suspensions showed aggregation by DLS at 923.4 nm, expected given the lower sulfate ester density on the surface. Thermally, SA:PA outperformed commercial CNC where Tmax rose to ∼340 °C, activation energy to 182 kJ/mol, and DSC transition to 316 °C. Phosphate monoester groups at the surface circumvent the dehydration pathways that typically lower degradation onset temperature.