The influence of temperature and moisture on the optical properties of cellulose in the presence of metal chlorides and glucuronic acid

摘要

The influence of Mg~(2+), Ca~(2+), Fe~(3+) ions and glucuronic acid on the dry and humid heat-induced (120-180℃) discoloration of cotton linter sheets was investigated. The quantity of the added metal ions was similar to that of metal ions found in wood and pulp. Heat-induced ageing (120-150℃, pH 5) under dry conditions caused only slight discoloration of the reference sheets and of the sheets impregnated with the metal ions. However, during ageing at 180℃, the presence of Mg~(2+) ions accelerated the discoloration. An initial pH in the range of 3-8 had no great influence on the discoloration of the reference sheets, but a low pH in combination with Mg~(2+) ions increased the discoloration drastically. Under humid ageing conditions at 180℃ ( < 30 min), the sheets discolored approximately twice as much as under dry conditions; the difference in discoloration caused by the metal ions was less than in dry ageing. Adding glucuronic acid to the sheets caused extensive discoloration under dry ageing conditions (120-180℃), while Ca~(2+) and Mg~(2+) ions decreased the glucuronic acid induced discoloration. Under humid conditions (180℃), however, Mg~(2+) and Ca~(2+) ions did not prevent the discoloration caused by glucuronic acid. An increase in the absorption coefficient was observed at 285 nm, most likely due to the formation of 5-hydroxymethyl-furfural (HMF) and 2-furaldehyde (furfural). GC-MS analysis reveals increased formation of HMF in the sheets impregnated with Mg~(2+) ions. Degradation of the cellulose in which the alkaline earth metals act as Lewis acids is proposed.