Is there a diffusion of alkali in the activation of dissolving cellulose pulp at low NAOH stoichiometric excess?

We conducted a quantitative study, following the degree of activation (i.e. the transformation to alkali cellulose, denoted as DoA) over time for dissolving cellulose pulp treated with different [NaOH] at low NaOH/anhydroglucose unit stoichiometric ratio (denoted as (r)2.6). Our quantitative approach was based on Raman spectroscopy data, evaluated by partial least squares regression modelling. The results show strong influence of the (r) on DoA (increasing from DoA=45% at (r)=0.8, to DoA=85% at (r)=2.6), and its complex dependence on [NaOH]. At (r)=0.8 the highest DoA (DoA60%) was found at 30% [NaOH], while at (r)=1.3 it was found at 20% [NaOH] (DoA80%). Although activation of cellulose happens in minutes at the studied temperature (30 degrees C), it was found that the reaction may be slow when a low (r) is used. A gradual increase of the DoA from approximate to 30 to approximate to 70% in time was seen when samples were activated with 30% [NaOH] at (r)=0.8. At the same (r), a similar increase of DoA from approximate to 30 to approximate to 60% was also observed when 40% [NaOH] was used. Slow diffusion of NaOH through poorly swollen cellulose fibres is proposed as an explanation for this phenomenon. Lastly, solid-state CP/MAS NMR measurements suggest that at a fixed temperature, the Na-Cell allomorph mostly depends on [NaOH]. However, in the transition area between Na-Cell I and Na-Cell II, its influence might be affected by (r). [GRAPHICS] .