Question

4. Pulping (heating wood with chemicals to remove lignin and separate the fibers) can be performed under alkaline or acidic conditions. In general alkaline conditions result in significant yield loss, but as long as you stay below 180°C the fiber is strong. Under acidic conditions yield is maintained better (about 2% higher), but the fiber is weak (these are fairly mild acidic conditions, so the cellulose is not reduced to oligo or monosugars). a.) Please explain what is happening to cellulose in these two systems and how this results in the fiber properties described (higher DP of cellulose results in stronger fiber). Explain the yield loss and or/strength loss. b.) what would happen if your reaction gets out of hand and your temperature rises to 200 °C? Why?

Answer 1

Pulping is a process of preparing cellulosic fibrous material by extracting cellulose fibres from wood by chemical reaction on wood, waste paper and other fibrous material. When wood is treated with alkaline solution like NaOH it depends on the tyoe of wood if it is hard wood then it will make the fibre more porous and will result in significant yield loss but the strength is more and in case of softwood it will decrease strength so the chemical treatment is performed under controlled temperature. When wood is treated with acid yield is greater because when sulfuric acid is used it is called sulphite process under which pure cellulosic fibre is obtained so yield is better and the fibre is also soft. Dp means degree of polymerization which is controlled according to the type of fibre needed. But in general reduction in dp is a necessary process to reduce tensile properties of fibre so if the dp is high then the fibre will be more strong due to the retention of greater tensile strength of fibre. If the temperature rises to 200 or more then cellulose fibre can deteriorate and lose its strength.