Kinetics of the production of chain-end groups and methanol from the depolymerization of cellulose during the ageing of paper/oil systems. Part 1: Standard wood kraft insulation

Abstract

Recently, the existence of a relation between the rupture of 1,4-β-glycosidic bonds in the cellulose during thermal-ageing of paper/oil systems and the detection of methanol in the oil has been reported for the first time in this journal (Jalbert et al. 2007). The present study addresses the rate constants of the reaction for standard wood kraft papers, two immersed in inhibited naphthenic oil under air (paper/oil weight–volume ratio of 1:18) and one in non-inhibited paraffinic oil under nitrogen (paper/oil weight–volume ratio of 1:30). The isotherms in the range of 60–130 °C show that the initial rate of methanol production markedly increases with temperature and to a lesser extent with the moisture of the specimens (initially between 0.5 and 2.25% (w/w)), similarly to what is noted for the depolymerization through the Ekenstam’s pseudo-zero order model. The Arrhenius expression of the rate constants reveals linear relationships that confirm the dominance of a given mechanism in both cases. A very good agreement is also noted for the activation energy over the entirely paper/oil systems studied (106.9 ± 4.3 and 103.5 ± 3.7 kJ mol⁻¹ for methanol and scissions, respectively). Furthermore, a comparison of the rate constants $\left( {k_{{{\text{CH}}_{ 3} {\text{OH}}}} /k_{\text{scissions}} } \right)$ shows approximately constant values indicating an apparent yield for the methanol of about one-third molecule per every scission for the tests under air (0.27 ± 0.04 for Clupak HD75 and 0.37 ± 0.14 for Munksjö TH70) and even lower for the ones under N₂ (0.12 ± 0.03 for Munksjö E.G.). As expected from a pseudo-zero order model, these values were shown to be consistent with a similar comparison of the amount of CH₃OH and chain-end groups produced under specific time–temperature ageing conditions (168 h at 120 °C). Finally, an additional test carried out with unaged cellulose in contact with a fresh solution of methanol in oil (cellulose/oil weight–volume ratio of 1:18) shows that at equilibrium, over 58% of the species is lost from the solution due to penetration into the fibres. Such results reveal the importance of the species partitioning in establishing the true correspondence between the molecules of CH₃OH produced and the scissions.