Preparation of cellulose and pulp carbamates through a reactive dissolution approach

Abstract

Carbamoylation of various cellulosic materials was systematically studied, using a reactive dissolution approach. Reactions with cellulose, or pulp and aromatic isocyanates, were initiated as heterogeneous mixtures in hot pyridine. As the reaction proceeded, homogeneous solutions were obtained. However, attempts to synthesize highly substituted cellulose carbamates, with aliphatic isocyanates in pyridine, failed as homogeneous solutions were not achieved, even after long reaction times. Consequently, aliphatic cellulose carbamates were prepared via reactive dissolution in dimethylacetamide (DMA), with dibutyltin dilaurate (DBTL) as catalyst. Reactions of cellulose with aliphatic isocyanates in DMA/DBTL at elevated temperature also ended up as homogeneous solutions. The success of the carbamoylation reaction was assessed by means of FTIR, ¹H, ¹³C, ³¹P and diffusion-ordered spectroscopy (DOSY) NMR. Carbamoylation of pre-hydrolysis hardwood kraft pulp (PHK), hardwood kraft pulp (HKP) and hemicellulose-poor hardwood kraft pulp (HPHKP) were also studied under similar conditions. Highly soluble carbamates were also obtained with the pulp derivatives. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the derivatives showed no significant improvement in the thermal properties compared with the untreated cellulose. However, DSC thermograms of some samples showed a phase change, which is attributed to a glass transition temperature (T_g). Also, a two-step decomposition process was observed in the TGA curves for the derivatives, which is attributed to the cleavage of carbamate substituents through a retro-carbamoylation and degradation of the cellulose backbone. Aromatic carbamates of cellulose showed similar molecular weight distributions compared to intact cellulose. However, for aliphatic cellulose carbamates the derivatization led to products with lower molecular weight.