Unveiling the structural characteristics of acetone-insoluble substances in various cellulose diacetate

Cellulose acetate (CA) is widely used in producing cellulose acetate tow, but the presence of acetone-insoluble substances (AIS) negatively affects dope filtration and product quality. This study systematically investigated AIS from industrial CA samples to elucidate their structural composition and formation mechanisms. Sugar composition analysis revealed that AIS primarily consists of insufficiently acetylated cellulose (glucose content: 70.51 %-78.00 %) and a minor fraction of acetylated hemicelluloses. Gel permeation chromatography (GPC) analysis showed that AIS exhibits a significantly higher absolute weight-average molecular weight (5.20-8.29 × 10⁴ g/mol) and dispersity index (3.57-4.51) than CA. X-ray diffraction (XRD) confirmed that AIS retains the crystalline structure of cellulose I, indicating incomplete acetylation. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) revealed that the degree of substitution of AIS (DS: 1.13-1.41) was much lower than that of CA (2.27-2.58). Furthermore, acetylated hemicellulose (HC-CA) exhibited poor acetone solubility and strong aggregation tendencies, highlighting its role in AIS formation. These findings suggest that AIS formation is primarily driven by incomplete acetylation of cellulose and the presence of insoluble acetylated hemicelluloses. This study provides insights for optimizing industrial CA production and minimizing AIS-related defects.